module ActiveRecord::FinderMethods

# File activerecord/lib/active_record/relation/finder_methods.rb, line 277
    def exists?(conditions = :none)
      if Base === conditions
        conditions = conditions.id
        ActiveSupport::Deprecation.warn("          You are passing an instance of ActiveRecord::Base to `exists?`.
          Please pass the id of the object by calling `.id`
".squish)
      end

      return false if !conditions

      relation = apply_join_dependency(self, construct_join_dependency)
      return false if ActiveRecord::NullRelation === relation

      relation = relation.except(:select, :order).select(ONE_AS_ONE).limit(1)

      case conditions
      when Array, Hash
        relation = relation.where(conditions)
      else
        unless conditions == :none
          relation = relation.where(primary_key => conditions)
        end
      end

      connection.select_value(relation, "#{name} Exists", relation.arel.bind_values + relation.bind_values) ? true : false
    end

Returns true if a record exists in the table that matches the id or conditions given, or false otherwise. The argument can take six forms:

• Integer - Finds the record with this primary key.

• String - Finds the record with a primary key corresponding to this string (such as '5').

• Array - Finds the record that matches these find-style conditions (such as ['name LIKE ?', "%#{query}%"]).

• Hash - Finds the record that matches these find-style conditions (such as {name: 'David'}).

• false - Returns always false.

• No args - Returns false if the table is empty, true otherwise.

For more information about specifying conditions as a hash or array, see the Conditions section in the introduction to ActiveRecord::Base.

Note: You can't pass in a condition as a string (like name = 'Jamie'), since it would be sanitized and then queried against the primary key column, like id = 'name = \'Jamie\''.

Person.exists?(5)
Person.exists?('5')
Person.exists?(['name LIKE ?', "%#{query}%"])
Person.exists?(id: [1, 4, 8])
Person.exists?(name: 'David')
Person.exists?(false)
Person.exists?

# File activerecord/lib/active_record/relation/finder_methods.rb, line 224
def fifth
  find_nth(4, offset_index)
end

Find the fifth record. If no order is defined it will order by primary key.

Person.fifth # returns the fifth object fetched by SELECT * FROM people
Person.offset(3).fifth # returns the fifth object from OFFSET 3 (which is OFFSET 7)
Person.where(["user_name = :u", { u: user_name }]).fifth

# File activerecord/lib/active_record/relation/finder_methods.rb, line 230
def fifth!
  find_nth! 4
end

Same as fifth but raises ActiveRecord::RecordNotFound if no record is found.

# File activerecord/lib/active_record/relation/finder_methods.rb, line 67
def find(*args)
  if block_given?
    to_a.find(*args) { |*block_args| yield(*block_args) }
  else
    find_with_ids(*args)
  end
end

Find by id - This can either be a specific id (1), a list of ids (1, 5, 6), or an array of ids ([5, 6, 10]). If no record can be found for all of the listed ids, then RecordNotFound will be raised. If the primary key is an integer, find by id coerces its arguments using to_i.

Person.find(1)          # returns the object for ID = 1
Person.find("1")        # returns the object for ID = 1
Person.find("31-sarah") # returns the object for ID = 31
Person.find(1, 2, 6)    # returns an array for objects with IDs in (1, 2, 6)
Person.find([7, 17])    # returns an array for objects with IDs in (7, 17)
Person.find([1])        # returns an array for the object with ID = 1
Person.where("administrator = 1").order("created_on DESC").find(1)

ActiveRecord::RecordNotFound will be raised if one or more ids are not found.

NOTE: The returned records may not be in the same order as the ids you provide since database rows are unordered. You'd need to provide an explicit order option if you want the results are sorted.

Find with lock

Example for find with a lock: Imagine two concurrent transactions: each will read person.visits == 2, add 1 to it, and save, resulting in two saves of person.visits = 3. By locking the row, the second transaction has to wait until the first is finished; we get the expected person.visits == 4.

Person.transaction do
  person = Person.lock(true).find(1)
  person.visits += 1
  person.save!
end

Variations of find

Person.where(name: 'Spartacus', rating: 4)
# returns a chainable list (which can be empty).

Person.find_by(name: 'Spartacus', rating: 4)
# returns the first item or nil.

Person.where(name: 'Spartacus', rating: 4).first_or_initialize
# returns the first item or returns a new instance (requires you call .save to persist against the database).

Person.where(name: 'Spartacus', rating: 4).first_or_create
# returns the first item or creates it and returns it, available since Rails 3.2.1.

Alternatives for find

Person.where(name: 'Spartacus', rating: 4).exists?(conditions = :none)
# returns a boolean indicating if any record with the given conditions exist.

Person.where(name: 'Spartacus', rating: 4).select("field1, field2, field3")
# returns a chainable list of instances with only the mentioned fields.

Person.where(name: 'Spartacus', rating: 4).ids
# returns an Array of ids, available since Rails 3.2.1.

Person.where(name: 'Spartacus', rating: 4).pluck(:field1, :field2)
# returns an Array of the required fields, available since Rails 3.1.

# File activerecord/lib/active_record/relation/finder_methods.rb, line 83
def find_by(*args)
  where(*args).take
rescue RangeError
  nil
end

Finds the first record matching the specified conditions. There is no implied ordering so if order matters, you should specify it yourself.

If no record is found, returns nil.

Post.find_by name: 'Spartacus', rating: 4
Post.find_by "published_at < ?", 2.weeks.ago

# File activerecord/lib/active_record/relation/finder_methods.rb, line 91
def find_by!(*args)
  where(*args).take!
rescue RangeError
  raise RecordNotFound, "Couldn't find #{@klass.name} with an out of range value"
end

Like find_by, except that if no record is found, raises an ActiveRecord::RecordNotFound error.

# File activerecord/lib/active_record/relation/finder_methods.rb, line 123
def first(limit = nil)
  if limit
    find_nth_with_limit(offset_index, limit)
  else
    find_nth(0, offset_index)
  end
end

Find the first record (or first N records if a parameter is supplied). If no order is defined it will order by primary key.

Person.first # returns the first object fetched by SELECT * FROM people ORDER BY people.id LIMIT 1
Person.where(["user_name = ?", user_name]).first
Person.where(["user_name = :u", { u: user_name }]).first
Person.order("created_on DESC").offset(5).first
Person.first(3) # returns the first three objects fetched by SELECT * FROM people ORDER BY people.id LIMIT 3

# File activerecord/lib/active_record/relation/finder_methods.rb, line 133
def first!
  find_nth! 0
end

Same as first but raises ActiveRecord::RecordNotFound if no record is found. Note that first! accepts no arguments.

# File activerecord/lib/active_record/relation/finder_methods.rb, line 240
def forty_two
  find_nth(41, offset_index)
end

Find the forty-second record. Also known as accessing “the reddit”. If no order is defined it will order by primary key.

Person.forty_two # returns the forty-second object fetched by SELECT * FROM people
Person.offset(3).forty_two # returns the forty-second object from OFFSET 3 (which is OFFSET 44)
Person.where(["user_name = :u", { u: user_name }]).forty_two

# File activerecord/lib/active_record/relation/finder_methods.rb, line 246
def forty_two!
  find_nth! 41
end

Same as forty_two but raises ActiveRecord::RecordNotFound if no record is found.

# File activerecord/lib/active_record/relation/finder_methods.rb, line 208
def fourth
  find_nth(3, offset_index)
end

Find the fourth record. If no order is defined it will order by primary key.

Person.fourth # returns the fourth object fetched by SELECT * FROM people
Person.offset(3).fourth # returns the fourth object from OFFSET 3 (which is OFFSET 6)
Person.where(["user_name = :u", { u: user_name }]).fourth

# File activerecord/lib/active_record/relation/finder_methods.rb, line 214
def fourth!
  find_nth! 3
end

Same as fourth but raises ActiveRecord::RecordNotFound if no record is found.

# File activerecord/lib/active_record/relation/finder_methods.rb, line 152
def last(limit = nil)
  if limit
    if order_values.empty? && primary_key
      order(arel_table[primary_key].desc).limit(limit).reverse
    else
      to_a.last(limit)
    end
  else
    find_last
  end
end

Find the last record (or last N records if a parameter is supplied). If no order is defined it will order by primary key.

Person.last # returns the last object fetched by SELECT * FROM people
Person.where(["user_name = ?", user_name]).last
Person.order("created_on DESC").offset(5).last
Person.last(3) # returns the last three objects fetched by SELECT * FROM people.

Take note that in that last case, the results are sorted in ascending order:

[#<Person id:2>, #<Person id:3>, #<Person id:4>]

and not:

[#<Person id:4>, #<Person id:3>, #<Person id:2>]

# File activerecord/lib/active_record/relation/finder_methods.rb, line 166
def last!
  last or raise RecordNotFound.new("Couldn't find #{@klass.name} with [#{arel.where_sql}]")
end

Same as last but raises ActiveRecord::RecordNotFound if no record is found. Note that last! accepts no arguments.

# File activerecord/lib/active_record/relation/finder_methods.rb, line 176
def second
  find_nth(1, offset_index)
end

Find the second record. If no order is defined it will order by primary key.

Person.second # returns the second object fetched by SELECT * FROM people
Person.offset(3).second # returns the second object from OFFSET 3 (which is OFFSET 4)
Person.where(["user_name = :u", { u: user_name }]).second

# File activerecord/lib/active_record/relation/finder_methods.rb, line 182
def second!
  find_nth! 1
end

Same as second but raises ActiveRecord::RecordNotFound if no record is found.

# File activerecord/lib/active_record/relation/finder_methods.rb, line 104
def take(limit = nil)
  limit ? limit(limit).to_a : find_take
end

Gives a record (or N records if a parameter is supplied) without any implied order. The order will depend on the database implementation. If an order is supplied it will be respected.

Person.take # returns an object fetched by SELECT * FROM people LIMIT 1
Person.take(5) # returns 5 objects fetched by SELECT * FROM people LIMIT 5
Person.where(["name LIKE '%?'", name]).take

# File activerecord/lib/active_record/relation/finder_methods.rb, line 110
def take!
  take or raise RecordNotFound.new("Couldn't find #{@klass.name} with [#{arel.where_sql}]")
end

Same as take but raises ActiveRecord::RecordNotFound if no record is found. Note that take! accepts no arguments.

# File activerecord/lib/active_record/relation/finder_methods.rb, line 192
def third
  find_nth(2, offset_index)
end

Find the third record. If no order is defined it will order by primary key.

Person.third # returns the third object fetched by SELECT * FROM people
Person.offset(3).third # returns the third object from OFFSET 3 (which is OFFSET 5)
Person.where(["user_name = :u", { u: user_name }]).third

# File activerecord/lib/active_record/relation/finder_methods.rb, line 198
def third!
  find_nth! 2
end

Same as third but raises ActiveRecord::RecordNotFound if no record is found.