# File activerecord/lib/active_record/attribute_methods.rb, line 322
def [](attr_name)
  read_attribute(attr_name) { |n| missing_attribute(n, caller) }
end

Returns the value of the attribute identified by attr_name after it has been typecast (for example, “2004-12-12” in a date column is cast to a date object, like Date.new(2004, 12, 12)). It raises ActiveModel::MissingAttributeError if the identified attribute is missing.

Note: :id is always present.

class Person < ActiveRecord::Base
  belongs_to :organization
end

person = Person.new(name: 'Francesco', age: '22')
person[:name] # => "Francesco"
person[:age]  # => 22

person = Person.select('id').first
person[:name]            # => ActiveModel::MissingAttributeError: missing attribute: name
person[:organization_id] # => ActiveModel::MissingAttributeError: missing attribute: organization_id

# File activerecord/lib/active_record/attribute_methods.rb, line 336
def []=(attr_name, value)
  write_attribute(attr_name, value)
end

Updates the attribute identified by attr_name with the specified value. (Alias for the protected write_attribute method).

class Person < ActiveRecord::Base
end

person = Person.new
person[:age] = '22'
person[:age] # => 22
person[:age].class # => Integer

# File activerecord/lib/active_record/attribute_methods.rb, line 369
def accessed_fields
  @attributes.accessed
end

Returns the name of all database fields which have been read from this model. This can be useful in development mode to determine which fields need to be selected. For performance critical pages, selecting only the required fields can be an easy performance win (assuming you aren't using all of the fields on the model).

For example:

class PostsController < ActionController::Base
  after_action :print_accessed_fields, only: :index

  def index
    @posts = Post.all
  end

  private

  def print_accessed_fields
    p @posts.first.accessed_fields
  end
end

Which allows you to quickly change your code to:

class PostsController < ActionController::Base
  def index
    @posts = Post.select(:id, :title, :author_id, :updated_at)
  end
end

# File activerecord/lib/active_record/attribute_methods.rb, line 280
def attribute_for_inspect(attr_name)
  value = _read_attribute(attr_name)
  format_for_inspect(value)
end

Returns an #inspect-like string for the value of the attribute attr_name. String attributes are truncated up to 50 characters, Date and Time attributes are returned in the :db format. Other attributes return the value of #inspect without modification.

person = Person.create!(name: 'David Heinemeier Hansson ' * 3)

person.attribute_for_inspect(:name)
# => "\"David Heinemeier Hansson David Heinemeier Hansson ...\""

person.attribute_for_inspect(:created_at)
# => "\"2012-10-22 00:15:07\""

person.attribute_for_inspect(:tag_ids)
# => "[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]"

# File activerecord/lib/active_record/attribute_methods.rb, line 248
def attribute_names
  @attributes.keys
end

Returns an array of names for the attributes available on this object.

class Person < ActiveRecord::Base
end

person = Person.new
person.attribute_names
# => ["id", "created_at", "updated_at", "name", "age"]

# File activerecord/lib/active_record/attribute_methods.rb, line 300
def attribute_present?(attribute)
  value = _read_attribute(attribute)
  !value.nil? && !(value.respond_to?(:empty?) && value.empty?)
end

Returns true if the specified attribute has been set by the user or by a database load and is neither nil nor empty? (the latter only applies to objects that respond to empty?, most notably Strings). Otherwise, false. Note that it always returns true with boolean attributes.

class Task < ActiveRecord::Base
end

task = Task.new(title: '', is_done: false)
task.attribute_present?(:title)   # => false
task.attribute_present?(:is_done) # => true
task.title = 'Buy milk'
task.is_done = true
task.attribute_present?(:title)   # => true
task.attribute_present?(:is_done) # => true

# File activerecord/lib/active_record/attribute_methods.rb, line 260
def attributes
  @attributes.to_hash
end

Returns a hash of all the attributes with their names as keys and the values of the attributes as values.

class Person < ActiveRecord::Base
end

person = Person.create(name: 'Francesco', age: 22)
person.attributes
# => {"id"=>3, "created_at"=>Sun, 21 Oct 2012 04:53:04, "updated_at"=>Sun, 21 Oct 2012 04:53:04, "name"=>"Francesco", "age"=>22}

# File activerecord/lib/active_record/attribute_methods.rb, line 236
def has_attribute?(attr_name)
  @attributes.key?(attr_name.to_s)
end

Returns true if the given attribute is in the attributes hash, otherwise false.

class Person < ActiveRecord::Base
end

person = Person.new
person.has_attribute?(:name)    # => true
person.has_attribute?('age')    # => true
person.has_attribute?(:nothing) # => false

# File activerecord/lib/active_record/attribute_methods.rb, line 211
def respond_to?(name, include_private = false)
  return false unless super

  # If the result is true then check for the select case.
  # For queries selecting a subset of columns, return false for unselected columns.
  # We check defined?(@attributes) not to issue warnings if called on objects that
  # have been allocated but not yet initialized.
  if defined?(@attributes)
    if name = self.class.symbol_column_to_string(name.to_sym)
      return has_attribute?(name)
    end
  end

  true
end

A Person object with a name attribute can ask person.respond_to?(:name), person.respond_to?(:name=), and person.respond_to?(:name?) which will all return true. It also defines the attribute methods if they have not been generated.

class Person < ActiveRecord::Base
end

person = Person.new
person.respond_to?(:name)    # => true
person.respond_to?(:name=)   # => true
person.respond_to?(:name?)   # => true
person.respond_to?('age')    # => true
person.respond_to?('age=')   # => true
person.respond_to?('age?')   # => true
person.respond_to?(:nothing) # => false