Collections

Meteor stores data in collections. To get started, declare a collection with new Mongo.Collection.

Anywhere

new Mongo.Collection(name, [options])

import { Mongo } from 'meteor/mongo' (mongo/collection.js, line 27)

Constructor for a Collection

Arguments

name String

The name of the collection. If null, creates an unmanaged (unsynchronized) local collection.

Options

connection Object

The server connection that will manage this collection. Uses the default connection if not specified. Pass the return value of calling DDP.connect to specify a different server. Pass null to specify no connection. Unmanaged (name is null) collections cannot specify a connection.

idGeneration String

The method of generating the _id fields of new documents in this collection. Possible values:

• 'STRING': random strings
• 'MONGO': random Mongo.ObjectID values

The default id generation technique is 'STRING'.

transform Function

An optional transformation function. Documents will be passed through this function before being returned from fetch or findOne, and before being passed to callbacks of observe, map, forEach, allow, and deny. Transforms are not applied for the callbacks of observeChanges or to cursors returned from publish functions.

defineMutationMethods Boolean

Set to false to skip setting up the mutation methods that enable insert/update/remove from client code. Default true.

Calling this function is analogous to declaring a model in a traditional ORM (Object-Relation Mapper)-centric framework. It sets up a collection (a storage space for records, or “documents”) that can be used to store a particular type of information, like users, posts, scores, todo items, or whatever matters to your application. Each document is a EJSON object. It includes an _id property whose value is unique in the collection, which Meteor will set when you first create the document.

// common code on client and server declares a DDP-managed mongo
// collection.
Chatrooms = new Mongo.Collection("chatrooms");
Messages = new Mongo.Collection("messages");

The function returns an object with methods to insert documents in the collection, update their properties, and remove them, and to find the documents in the collection that match arbitrary criteria. The way these methods work is compatible with the popular Mongo database API. The same database API works on both the client and the server (see below).

// return array of my messages

var myMessages = Messages.find({userId: Session.get('myUserId')}).fetch();

// create a new message
Messages.insert({text: "Hello, world!"});

// mark my first message as "important"
Messages.update(myMessages[0]._id, {$set: {important: true}});

If you pass a name when you create the collection, then you are declaring a persistent collection — one that is stored on the server and seen by all users. Client code and server code can both access the same collection using the same API.

Specifically, when you pass a name, here’s what happens:

• On the server (if you do not specify a connection), a collection with that name is created on a backend Mongo server. When you call methods on that collection on the server, they translate directly into normal Mongo operations (after checking that they match your access control rules).

• On the client (and on the server if you specify a connection), a Minimongo instance is created. Minimongo is essentially an in-memory, non-persistent implementation of Mongo in pure JavaScript. It serves as a local cache that stores just the subset of the database that this client is working with. Queries (find) on these collections are served directly out of this cache, without talking to the server.

• When you write to the database on the client (insert, update, remove), the command is executed locally immediately, and, simultaneously, it’s sent to the server and executed there too. This happens via stubs, because writes are implemented as methods.

When, on the server, you write to a collection which has a specified connection to another server, it sends the corresponding method to the other server and receives the changed values back from it over DDP. Unlike on the client, it does not execute the write locally first.

If you pass null as the name, then you’re creating a local collection. It’s not synchronized anywhere; it’s just a local scratchpad that supports Mongo-style find, insert, update, and remove operations. (On both the client and the server, this scratchpad is implemented using Minimongo.)

By default, Meteor automatically publishes every document in your collection to each connected client. To turn this behavior off, remove the autopublish package, in your terminal:

meteor remove autopublish

and instead call Meteor.publish to specify which parts of your collection should be published to which users.

// Create a collection called Posts and put a document in it. The
// document will be immediately visible in the local copy of the
// collection. It will be written to the server-side database
// a fraction of a second later, and a fraction of a second
// after that, it will be synchronized down to any other clients
// that are subscribed to a query that includes it (see
// Meteor.subscribe and autopublish)
Posts = new Mongo.Collection("posts");
Posts.insert({title: "Hello world", body: "First post"});

// Changes are visible immediately -- no waiting for a round trip to
// the server.
assert(Posts.find().count() === 1);

// Create a temporary, local collection. It works just like any other
// collection, but it doesn't send changes to the server, and it
// can't receive any data from subscriptions.
Scratchpad = new Mongo.Collection;

for (var i = 0; i < 10; i++)
  Scratchpad.insert({number: i * 2});
assert(Scratchpad.find({number: {$lt: 9}}).count() === 5);

Generally, you’ll assign Mongo.Collection objects in your app to global variables. You can only create one Mongo.Collection object for each underlying Mongo collection.

If you specify a transform option to the Collection or any of its retrieval methods, documents are passed through the transform function before being returned or passed to callbacks. This allows you to add methods or otherwise modify the contents of your collection from their database representation. You can also specify transform on a particular find, findOne, allow, or deny call. Transform functions must return an object and they may not change the value of the document’s _id field (though it’s OK to leave it out).

// An Animal class that takes a document in its constructor
Animal = function (doc) {
  _.extend(this, doc);
};
_.extend(Animal.prototype, {
  makeNoise: function () {
    console.log(this.sound);
  }
});

// Define a Collection that uses Animal as its document
Animals = new Mongo.Collection("Animals", {
  transform: function (doc) { return new Animal(doc); }
});

// Create an Animal and call its makeNoise method
Animals.insert({name: "raptor", sound: "roar"});
Animals.findOne({name: "raptor"}).makeNoise(); // prints "roar"

transform functions are not called reactively. If you want to add a dynamically changing attribute to an object, do it with a function that computes the value at the time it’s called, not by computing the attribute at transform time.

In this release, Minimongo has some limitations:

• $pull in modifiers can only accept certain kinds of selectors.
• findAndModify, aggregate functions, and map/reduce aren’t supported.

All of these will be addressed in a future release. For full Minimongo release notes, see packages/minimongo/NOTES in the repository.

Minimongo doesn’t currently have indexes. It’s rare for this to be an issue, since it’s unusual for a client to have enough data that an index is worthwhile.

Read more about collections and how to use them in the Collections article in the Meteor Guide.

Anywhere

Mongo.Collection#find([selector], [options])

(mongo/collection.js, line 290)

Find the documents in a collection that match the selector.

Arguments

selector Mongo Selector, Object ID, or String

A query describing the documents to find

Options

sort Mongo Sort Specifier

Sort order (default: natural order)

skip Number

Number of results to skip at the beginning

limit Number

Maximum number of results to return

fields Mongo Field Specifier

Dictionary of fields to return or exclude.

reactive Boolean

(Client only) Default true; pass false to disable reactivity

transform Function

Overrides transform on the Collection for this cursor. Pass null to disable transformation.

disableOplog Boolean

(Server only) Pass true to disable oplog-tailing on this query. This affects the way server processes calls to observe on this query. Disabling the oplog can be useful when working with data that updates in large batches.

pollingIntervalMs Number

(Server only) How often to poll this query when observing on the server. In milliseconds. Defaults to 10 seconds.

pollingThrottleMs Number

(Server only) Minimum time to allow between re-polling. Increasing this will save CPU and mongo load at the expense of slower updates to users. Decreasing this is not recommended. In milliseconds. Defaults to 50 milliseconds.

find returns a cursor. It does not immediately access the database or return documents. Cursors provide fetch to return all matching documents, map and forEach to iterate over all matching documents, and observe and observeChanges to register callbacks when the set of matching documents changes.

Collection cursors are not query snapshots. If the database changes between calling Collection.find and fetching the results of the cursor, or while fetching results from the cursor, those changes may or may not appear in the result set.

Cursors are a reactive data source. On the client, the first time you retrieve a cursor’s documents with fetch, map, or forEach inside a reactive computation (eg, a template or autorun), Meteor will register a dependency on the underlying data. Any change to the collection that changes the documents in a cursor will trigger a recomputation. To disable this behavior, pass {reactive: false} as an option to find.

Note that when fields are specified, only changes to the included fields will trigger callbacks in observe, observeChanges and invalidations in reactive computations using this cursor. Careful use of fields allows for more fine-grained reactivity for computations that don’t depend on an entire document.

On the client, there will be a period of time between when the page loads and when the published data arrives from the server during which your client-side collections will be empty.

Anywhere

Mongo.Collection#findOne([selector], [options])

(mongo/collection.js, line 319)

Finds the first document that matches the selector, as ordered by sort and skip options.

Arguments

selector Mongo Selector, Object ID, or String

A query describing the documents to find

Options

sort Mongo Sort Specifier

Sort order (default: natural order)

skip Number

Number of results to skip at the beginning

fields Mongo Field Specifier

Dictionary of fields to return or exclude.

reactive Boolean

(Client only) Default true; pass false to disable reactivity

transform Function

Overrides transform on the Collection for this cursor. Pass null to disable transformation.

Equivalent to find(selector, options).fetch()[0] with options.limit = 1.

Anywhere

Mongo.Collection#insert(doc, [callback])

(mongo/collection.js, line 458)

Insert a document in the collection. Returns its unique _id.

Arguments

doc Object

The document to insert. May not yet have an _id attribute, in which case Meteor will generate one for you.

callback Function

Optional. If present, called with an error object as the first argument and, if no error, the _id as the second.

Add a document to the collection. A document is just an object, and its fields can contain any combination of EJSON-compatible datatypes (arrays, objects, numbers, strings, null, true, and false).

insert will generate a unique ID for the object you pass, insert it in the database, and return the ID. When insert is called from untrusted client code, it will be allowed only if passes any applicable allow and deny rules.

On the server, if you don’t provide a callback, then insert blocks until the database acknowledges the write, or throws an exception if something went wrong. If you do provide a callback, insert still returns the ID immediately. Once the insert completes (or fails), the callback is called with error and result arguments. In an error case, result is undefined. If the insert is successful, error is undefined and result is the new document ID.

On the client, insert never blocks. If you do not provide a callback and the insert fails on the server, then Meteor will log a warning to the console. If you provide a callback, Meteor will call that function with error and result arguments. In an error case, result is undefined. If the insert is successful, error is undefined and result is the new document ID.

Example:

var groceriesId = Lists.insert({name: "Groceries"});
Items.insert({list: groceriesId, name: "Watercress"});
Items.insert({list: groceriesId, name: "Persimmons"});

Anywhere

Mongo.Collection#update(selector, modifier, [options], [callback])

(mongo/collection.js, line 539)

Modify one or more documents in the collection. Returns the number of affected documents.

Arguments

selector Mongo Selector, Object ID, or String

Specifies which documents to modify

modifier Mongo Modifier

Specifies how to modify the documents

callback Function

Optional. If present, called with an error object as the first argument and, if no error, the number of affected documents as the second.

Options

multi Boolean

True to modify all matching documents; false to only modify one of the matching documents (the default).

upsert Boolean

True to insert a document if no matching documents are found.

Modify documents that match selector according to modifier (see modifier documentation).

The behavior of update differs depending on whether it is called by trusted or untrusted code. Trusted code includes server code and method code. Untrusted code includes client-side code such as event handlers and a browser’s JavaScript console.

• Trusted code can modify multiple documents at once by setting multi to true, and can use an arbitrary Mongo selector to find the documents to modify. It bypasses any access control rules set up by allow and deny. The number of affected documents will be returned from the update call if you don’t pass a callback.

• Untrusted code can only modify a single document at once, specified by its _id. The modification is allowed only after checking any applicable allow and deny rules. The number of affected documents will be returned to the callback. Untrusted code cannot perform upserts, except in insecure mode.

On the server, if you don’t provide a callback, then update blocks until the database acknowledges the write, or throws an exception if something went wrong. If you do provide a callback, update returns immediately. Once the update completes, the callback is called with a single error argument in the case of failure, or a second argument indicating the number of affected documents if the update was successful.

On the client, update never blocks. If you do not provide a callback and the update fails on the server, then Meteor will log a warning to the console. If you provide a callback, Meteor will call that function with an error argument if there was an error, or a second argument indicating the number of affected documents if the update was successful.

Client example:

// When the givePoints button in the admin dashboard is pressed,
// give 5 points to the current player. The new score will be
// immediately visible on everyone's screens.
Template.adminDashboard.events({
  'click .givePoints': function () {
    Players.update(Session.get("currentPlayer"), {$inc: {score: 5}});
  }
});

Server example:

// Give the "Winner" badge to each user with a score greater than
// 10. If they are logged in and their badge list is visible on the
// screen, it will update automatically as they watch.
Meteor.methods({
  declareWinners: function () {
    Players.update({score: {$gt: 10}},
                   {$addToSet: {badges: "Winner"}},
                   {multi: true});
  }
});

You can use update to perform a Mongo upsert by setting the upsert option to true. You can also use the upsert method to perform an upsert that returns the _id of the document that was inserted (if there was one) in addition to the number of affected documents.

Anywhere

Mongo.Collection#upsert(selector, modifier, [options], [callback])

(mongo/collection.js, line 664)

Modify one or more documents in the collection, or insert one if no matching documents were found. Returns an object with keys numberAffected (the number of documents modified) and insertedId (the unique _id of the document that was inserted, if any).

Arguments

selector Mongo Selector, Object ID, or String

Specifies which documents to modify

modifier Mongo Modifier

Specifies how to modify the documents

callback Function

Optional. If present, called with an error object as the first argument and, if no error, the number of affected documents as the second.

Options

multi Boolean

True to modify all matching documents; false to only modify one of the matching documents (the default).

Modify documents that match selector according to modifier, or insert a document if no documents were modified. upsert is the same as calling update with the upsert option set to true, except that the return value of upsert is an object that contain the keys numberAffected and insertedId. (update returns only the number of affected documents.)

Anywhere

Mongo.Collection#remove(selector, [callback])

(mongo/collection.js, line 600)

Remove documents from the collection

Arguments

selector Mongo Selector, Object ID, or String

Specifies which documents to remove

callback Function

Optional. If present, called with an error object as its argument.

Find all of the documents that match selector and delete them from the collection.

The behavior of remove differs depending on whether it is called by trusted or untrusted code. Trusted code includes server code and method code. Untrusted code includes client-side code such as event handlers and a browser’s JavaScript console.

• Trusted code can use an arbitrary Mongo selector to find the documents to remove, and can remove more than one document at once by passing a selector that matches multiple documents. It bypasses any access control rules set up by allow and deny. The number of removed documents will be returned from remove if you don’t pass a callback.

  As a safety measure, if selector is omitted (or is undefined), no documents will be removed. Set selector to {} if you really want to remove all documents from your collection.

• Untrusted code can only remove a single document at a time, specified by its _id. The document is removed only after checking any applicable allow and deny rules. The number of removed documents will be returned to the callback.

On the server, if you don’t provide a callback, then remove blocks until the database acknowledges the write and then returns the number of removed documents, or throws an exception if something went wrong. If you do provide a callback, remove returns immediately. Once the remove completes, the callback is called with a single error argument in the case of failure, or a second argument indicating the number of removed documents if the remove was successful.

On the client, remove never blocks. If you do not provide a callback and the remove fails on the server, then Meteor will log a warning to the console. If you provide a callback, Meteor will call that function with an error argument if there was an error, or a second argument indicating the number of removed documents if the remove was successful.

Client example:

// When the remove button is clicked on a chat message, delete
// that message.
Template.chat.events({
  'click .remove': function () {
    Messages.remove(this._id);
  }
});

Server example:

// When the server starts, clear the log, and delete all players
// with a karma of less than -2.
Meteor.startup(function () {
  if (Meteor.isServer) {
    Logs.remove({});
    Players.remove({karma: {$lt: -2}});
  }
});

Server

Mongo.Collection#allow(options)

(allow-deny/allow-deny.js, line 43)

Allow users to write directly to this collection from client code, subject to limitations you define.

Options

insert, update, remove Function

Functions that look at a proposed modification to the database and return true if it should be allowed.

fetch Array of Strings

Optional performance enhancement. Limits the fields that will be fetched from the database for inspection by your update and remove functions.

transform Function

Overrides transform on the Collection. Pass null to disable transformation.

When a client calls insert, update, or remove on a collection, the collection’s allow and deny callbacks are called on the server to determine if the write should be allowed. If at least one allow callback allows the write, and no deny callbacks deny the write, then the write is allowed to proceed.

These checks are run only when a client tries to write to the database directly, for example by calling update from inside an event handler. Server code is trusted and isn’t subject to allow and deny restrictions. That includes methods that are called with Meteor.call — they are expected to do their own access checking rather than relying on allow and deny.

You can call allow as many times as you like, and each call can include any combination of insert, update, and remove functions. The functions should return true if they think the operation should be allowed. Otherwise they should return false, or nothing at all (undefined). In that case Meteor will continue searching through any other allow rules on the collection.

The available callbacks are:

insert(userId, doc)

The user userId wants to insert the document doc into the collection. Return true if this should be allowed.

doc will contain the _id field if one was explicitly set by the client, or if there is an active transform. You can use this to prevent users from specifying arbitrary _id fields.

update(userId, doc, fieldNames, modifier)

The user userId wants to update a document doc. (doc is the current version of the document from the database, without the proposed update.) Return true to permit the change.

fieldNames is an array of the (top-level) fields in doc that the client wants to modify, for example ['name', 'score'].

modifier is the raw Mongo modifier that the client wants to execute; for example, {$set: {'name.first': "Alice"}, $inc: {score: 1}}.

Only Mongo modifiers are supported (operations like $set and $push). If the user tries to replace the entire document rather than use $-modifiers, the request will be denied without checking the allow functions.

remove(userId, doc)

The user userId wants to remove doc from the database. Return true to permit this.

When calling update or remove Meteor will by default fetch the entire document doc from the database. If you have large documents you may wish to fetch only the fields that are actually used by your functions. Accomplish this by setting fetch to an array of field names to retrieve.

Example:

// Create a collection where users can only modify documents that
// they own. Ownership is tracked by an 'owner' field on each
// document. All documents must be owned by the user that created
// them and ownership can't be changed. Only a document's owner
// is allowed to delete it, and the 'locked' attribute can be
// set on a document to prevent its accidental deletion.

Posts = new Mongo.Collection("posts");

Posts.allow({
  insert: function (userId, doc) {
    // the user must be logged in, and the document must be owned by the user

    return (userId && doc.owner === userId);
  },
  update: function (userId, doc, fields, modifier) {
    // can only change your own documents

    return doc.owner === userId;
  },
  remove: function (userId, doc) {
    // can only remove your own documents

    return doc.owner === userId;
  },
  fetch: ['owner']
});

Posts.deny({
  update: function (userId, doc, fields, modifier) {
    // can't change owners

    return _.contains(fields, 'owner');
  },
  remove: function (userId, doc) {
    // can't remove locked documents

    return doc.locked;
  },
  fetch: ['locked'] // no need to fetch 'owner'

});

If you never set up any allow rules on a collection then all client writes to the collection will be denied, and it will only be possible to write to the collection from server-side code. In this case you will have to create a method for each possible write that clients are allowed to do. You’ll then call these methods with Meteor.call rather than having the clients call insert, update, and remove directly on the collection.

Meteor also has a special “insecure mode” for quickly prototyping new applications. In insecure mode, if you haven’t set up any allow or deny rules on a collection, then all users have full write access to the collection. This is the only effect of insecure mode. If you call allow or deny at all on a collection, even Posts.allow({}), then access is checked just like normal on that collection. New Meteor projects start in insecure mode by default. To turn it off just run in your terminal:

meteor remove insecure

Server

Mongo.Collection#deny(options)

(allow-deny/allow-deny.js, line 58)

Override allow rules.

Options

insert, update, remove Function

Functions that look at a proposed modification to the database and return true if it should be denied, even if an allow rule says otherwise.

fetch Array of Strings

Optional performance enhancement. Limits the fields that will be fetched from the database for inspection by your update and remove functions.

transform Function

Overrides transform on the Collection. Pass null to disable transformation.

This works just like allow, except it lets you make sure that certain writes are definitely denied, even if there is an allow rule that says that they should be permitted.

When a client tries to write to a collection, the Meteor server first checks the collection’s deny rules. If none of them return true then it checks the collection’s allow rules. Meteor allows the write only if no deny rules return true and at least one allow rule returns true.

Server

Mongo.Collection#rawCollection()

(mongo/collection.js, line 710)

Returns the Collection object corresponding to this collection from the npm mongodb driver module which is wrapped by Mongo.Collection.

Server

Mongo.Collection#rawDatabase()

(mongo/collection.js, line 722)

Returns the Db object corresponding to this collection's database connection from the npm mongodb driver module which is wrapped by Mongo.Collection.

Cursors

To create a cursor, use find. To access the documents in a cursor, use forEach, map, or fetch.

Anywhere

Mongo.Cursor#forEach(callback, [thisArg])

(minimongo/minimongo.js, line 145)

Call callback once for each matching document, sequentially and synchronously.

Arguments

callback Function

Function to call. It will be called with three arguments: the document, a 0-based index, and cursor itself.

thisArg Any

An object which will be the value of this inside callback.

This interface is compatible with Array.forEach.

When called from a reactive computation, forEach registers dependencies on the matching documents.

Examples:

// Print the titles of the five top-scoring posts

var topPosts = Posts.find({}, {sort: {score: -1}, limit: 5});

var count = 0;
topPosts.forEach(function (post) {
  console.log("Title of post " + count + ": " + post.title);
  count += 1;
});

Anywhere

Mongo.Cursor#map(callback, [thisArg])

(minimongo/minimongo.js, line 181)

Map callback over all matching documents. Returns an Array.

Arguments

callback Function

Function to call. It will be called with three arguments: the document, a 0-based index, and cursor itself.

thisArg Any

An object which will be the value of this inside callback.

This interface is compatible with Array.map.

When called from a reactive computation, map registers dependencies on the matching documents.

On the server, if callback yields, other calls to callback may occur while the first call is waiting. If strict sequential execution is necessary, use forEach instead.

Anywhere

Mongo.Cursor#fetch()

(minimongo/minimongo.js, line 199)

Return all matching documents as an Array.

When called from a reactive computation, fetch registers dependencies on the matching documents.

Anywhere

Mongo.Cursor#count()

(minimongo/minimongo.js, line 216)

Returns the number of documents that match a query.

Unlike the other functions, count registers a dependency only on the number of matching documents. (Updates that just change or reorder the documents in the result set will not trigger a recomputation.)

Anywhere

Mongo.Cursor#observe(callbacks)

(minimongo/minimongo.js, line 303)

Watch a query. Receive callbacks as the result set changes.

Arguments

callbacks Object

Functions to call to deliver the result set as it changes

Establishes a live query that invokes callbacks when the result of the query changes. The callbacks receive the entire contents of the document that was affected, as well as its old contents, if applicable. If you only need to receive the fields that changed, see observeChanges.

callbacks may have the following functions as properties:

added(document) or

addedAt(document, atIndex, before)

A new document document entered the result set. The new document appears at position atIndex. It is immediately before the document whose _id is before. before will be null if the new document is at the end of the results.

changed(newDocument, oldDocument) or

changedAt(newDocument, oldDocument, atIndex)

The contents of a document were previously oldDocument and are now newDocument. The position of the changed document is atIndex.

removed(oldDocument) or

removedAt(oldDocument, atIndex)

The document oldDocument is no longer in the result set. It used to be at position atIndex.

movedTo(document, fromIndex, toIndex, before)

A document changed its position in the result set, from fromIndex to toIndex (which is before the document with id before). Its current contents is document.

Use added, changed, and removed when you don’t care about the order of the documents in the result set. They are more efficient than addedAt, changedAt, and removedAt.

Before observe returns, added (or addedAt) will be called zero or more times to deliver the initial results of the query.

observe returns a live query handle, which is an object with a stop method. Call stop with no arguments to stop calling the callback functions and tear down the query. The query will run forever until you call this. If observe is called from a Tracker.autorun computation, it is automatically stopped when the computation is rerun or stopped. (If the cursor was created with the option reactive set to false, it will only deliver the initial results and will not call any further callbacks; it is not necessary to call stop on the handle.)

Anywhere

Mongo.Cursor#observeChanges(callbacks)

(minimongo/minimongo.js, line 315)

Watch a query. Receive callbacks as the result set changes. Only the differences between the old and new documents are passed to the callbacks.

Arguments

callbacks Object

Functions to call to deliver the result set as it changes

Establishes a live query that invokes callbacks when the result of the query changes. In contrast to observe, observeChanges provides only the difference between the old and new result set, not the entire contents of the document that changed.

callbacks may have the following functions as properties:

added(id, fields) or

addedBefore(id, fields, before)

A new document entered the result set. It has the id and fields specified. fields contains all fields of the document excluding the _id field. The new document is before the document identified by before, or at the end if before is null.

changed(id, fields)

The document identified by id has changed. fields contains the changed fields with their new values. If a field was removed from the document then it will be present in fields with a value of undefined.

movedBefore(id, before)

The document identified by id changed its position in the ordered result set, and now appears before the document identified by before.

removed(id)

The document identified by id was removed from the result set.

observeChanges is significantly more efficient if you do not use addedBefore or movedBefore.

Before observeChanges returns, added (or addedBefore) will be called zero or more times to deliver the initial results of the query.

observeChanges returns a live query handle, which is an object with a stop method. Call stop with no arguments to stop calling the callback functions and tear down the query. The query will run forever until you call this. If observeChanges is called from a Tracker.autorun computation, it is automatically stopped when the computation is rerun or stopped. (If the cursor was created with the option reactive set to false, it will only deliver the initial results and will not call any further callbacks; it is not necessary to call stop on the handle.)

Unlike observe, observeChanges does not provide absolute position information (that is, atIndex positions rather than before positions.) This is for efficiency.

Example:

// Keep track of how many administrators are online.

var count = 0;

var query = Users.find({admin: true, onlineNow: true});

var handle = query.observeChanges({
  added: function (id, user) {
    count++;
    console.log(user.name + " brings the total to " + count + " admins.");
  },
  removed: function () {
    count--;
    console.log("Lost one. We're now down to " + count + " admins.");
  }
});

// After five seconds, stop keeping the count.
setTimeout(function () {handle.stop();}, 5000);

Anywhere

new Mongo.ObjectID([hexString])

import { Mongo } from 'meteor/mongo' (mongo/collection.js, line 737)

Create a Mongo-style ObjectID. If you don't specify a hexString, the ObjectID will generated randomly (not using MongoDB's ID construction rules).

Arguments

hexString String

Optional. The 24-character hexadecimal contents of the ObjectID to create

Mongo.ObjectID follows the same API as the Node MongoDB driver ObjectID class. Note that you must use the equals method (or EJSON.equals) to compare them; the === operator will not work. If you are writing generic code that needs to deal with _id fields that may be either strings or ObjectIDs, use EJSON.equals instead of === to compare them.

ObjectID values created by Meteor will not have meaningful answers to their getTimestamp method, since Meteor currently constructs them fully randomly.

Mongo-Style Selectors

The simplest selectors are just a string or Mongo.ObjectID. These selectors match the document with that value in its _id field.

A slightly more complex form of selector is an object containing a set of keys that must match in a document:

// Matches all documents where deleted is false
{deleted: false}

// Matches all documents where the name and cognomen are as given
{name: "Rhialto", cognomen: "the Marvelous"}

// Matches every document
{}

But they can also contain more complicated tests:

// Matches documents where age is greater than 18
{age: {$gt: 18}}

// Also matches documents where tags is an array containing "popular"
{tags: "popular"}

// Matches documents where fruit is one of three possibilities
{fruit: {$in: ["peach", "plum", "pear"]}}

See the complete documentation.

Mongo-Style Modifiers

A modifier is an object that describes how to update a document in place by changing some of its fields. Some examples:

// Set the 'admin' property on the document to true
{$set: {admin: true}}

// Add 2 to the 'votes' property, and add "Traz"
// to the end of the 'supporters' array
{$inc: {votes: 2}, $push: {supporters: "Traz"}}

But if a modifier doesn’t contain any $-operators, then it is instead interpreted as a literal document, and completely replaces whatever was previously in the database. (Literal document modifiers are not currently supported by validated updates.)

// Find the document with id "123", and completely replace it.
Users.update({_id: "123"}, {name: "Alice", friends: ["Bob"]});

See the full list of modifiers.

Sort Specifiers

Sorts may be specified using your choice of several syntaxes:

// All of these do the same thing (sort in ascending order by

// key "a", breaking ties in descending order of key "b")

[["a", "asc"], ["b", "desc"]]
["a", ["b", "desc"]]
{a: 1, b: -1}

The last form will only work if your JavaScript implementation preserves the order of keys in objects. Most do, most of the time, but it’s up to you to be sure.

For local collections you can pass a comparator function which receives two document objects, and returns -1 if the first document comes first in order, 1 if the second document comes first, or 0 if neither document comes before the other. This is a Minimongo extension to MongoDB.

Field Specifiers

Queries can specify a particular set of fields to include or exclude from the result object.

To exclude specific fields from the result objects, the field specifier is a dictionary whose keys are field names and whose values are 0. All unspecified fields are included.

Users.find({}, {fields: {password: 0, hash: 0}})

To include only specific fields in the result documents, use 1 as the value. The _id field is still included in the result.

Users.find({}, {fields: {firstname: 1, lastname: 1}})

With one exception, it is not possible to mix inclusion and exclusion styles: the keys must either be all 1 or all 0. The exception is that you may specify _id: 0 in an inclusion specifier, which will leave _id out of the result object as well. However, such field specifiers can not be used with observeChanges, observe, cursors returned from a publish function, or cursors used in {% raw %}{{#each}}{% endraw %} in a template. They may be used with fetch, findOne, forEach, and map.

Field operators such as $ and $elemMatch are not available on the client side yet.

A more advanced example:

Users.insert({ alterEgos: [{ name: "Kira", alliance: "murderer" },
                           { name: "L", alliance: "police" }],
               name: "Yagami Light" });

Users.findOne({}, { fields: { 'alterEgos.name': 1, _id: 0 } });

// returns { alterEgos: [{ name: "Kira" }, { name: "L" }] }

See the MongoDB docs for details of the nested field rules and array behavior.