CRUD Operations

Saving Documents

Mongoid supports all expected CRUD operations for those familiar with other Ruby mappers like Active Record or Data Mapper. What distinguishes Mongoid from other mappers for MongoDB is that the general persistence operations perform atomic updates on only the fields that have changed instead of writing the entire document to the database each time.

The persistence sections will provide examples on what database operation is performed when executing the documented command.

Standard

Mongoid's standard persistence methods come in the form of common methods you would find in other mapping frameworks. The following table shows all standard operations with examples.

Mongoid provides the following persistence-related attributes:

Atomic

Mongoid exposes MongoDB update operators </reference/operator/update/> as methods on Mongoid documents. When these methods are used, callbacks are not invoked and validations are not performed. The supported update operators are:

Note that, because these methods skip validations, it is possible to both save invalid documents into the database and end up with invalid documents in the application (which would subsequently fail to save via a save call due to the failing validations).

Atomic Operation Grouping

Atomic operations may be grouped together using the #atomically method on a document. All operations inside the block given to #atomically are sent to the cluster in a single atomic command. For example:

person.atomically do
  person.inc(age: 1)
  person.set(name: 'Jake')
end

#atomically blocks may be nested. The default behavior is to write changes performed by each block as soon as the block ends:

person.atomically do
  person.atomically do
    person.inc(age: 1)
    person.set(name: 'Jake')
  end
  raise 'An exception'
  # name and age changes are still persisted
end

This behavior can be changed by specifying the join_context: true option to #atomically, or globally by setting the join_contexts configuration option <configuration-options> to true. When context joining is enabled, nested #atomically blocks are joined with the outer blocks, and only the outermost block (or the first block where join_contexts is false) actually writes changes to the cluster. For example:

person.atomically do
  person.atomically(join_context: true) do
    person.inc(age: 1)
    person.set(name: 'Jake')
  end
  raise 'An exception'
  # name and age changes are not persisted
end

The context joining behavior can be enabled globally by default by setting join_context option in Mongoid configuration. In this case specifying join_context: false on an #atomically block can be used to obtain the independent persistence context behavior.

If an exception is raised in an #atomically block which has not yet persisted its changes to the cluster, any pending attribute changes on Mongoid models are reverted. For example:

person = Person.new(name: 'Tom')
begin
  person.atomically do
    person.inc(age: 1)
    person.set(name: 'Jake')
    person.name # => 'Jake'
    raise 'An exception'
  end
rescue Exception
  person.name # => 'Tom'
end

Atomic operations described in this section apply to one document at a time, therefore nesting #atomically blocks invoked on multiple documents does not make changes to the different documents be persisted atomically together. However, MongoDB offers multi-document transactions <transactions> as of server version 4.0 which provide atomic persistence across multiple documents.

Reloading

Use the reload method to fetch the most recent version of a document from the database. Any unsaved modifications to the document's attributes are lost:

band = Band.create!(name: 'foo')
# => #<Band _id: 6206d06de1b8324561f179c9, name: "foo", description: nil, likes: nil>

band.name = 'bar'
band
# => #<Band _id: 6206d06de1b8324561f179c9, name: "bar", description: nil, likes: nil>

band.reload
# => #<Band _id: 6206d06de1b8324561f179c9, name: "foo", description: nil, likes: nil>

When a document is reloaded, all of its embedded associations are also reloaded in the same query (since embedded documents are stored in the parent document on the server). If a document has referenced associations, the loaded associations' are not reloaded but their values are cleared, such that these associations would be loaded from the database at the next access.

# Assuming band has many tours, which could be referenced:
band = Band.create!(tours: [Tour.create!])
# ... or embedded:
band = Band.create!(tours: [Tour.new])

# This writes the empty tour list into the database.
band.tours = []

# There are no unsaved modifications in band at this point to be reverted.
band.reload

# Returns the empty array since this is what is in the database.
band.tours
# => []

If the model has a shard key <shard-keys> defined, the shard key value is included in the reloading query.

If the database does not contain a matching document, Mongoid normally raises ::Mongoid::Errors::DocumentNotFound. However, if the configuration option raise_not_found_error is set to false, and the database does not contain a matching document, Mongoid replaces the current document with a newly created document whose attributes are set to default values. Importantly, this generally causes the _id of the document to change, as the following example demonstrates:

band = Band.create!
# => #<Band _id: 6206d00de1b8324561f179c7, name: "foo", description: nil, likes: nil>

Mongoid.raise_not_found_error = false
band.destroy

band.reload
# => #<Band _id: 6206d031e1b8324561f179c8, name: nil, description: nil, likes: nil>

For this reason, it is not recommended to use reload when raise_not_found_error is set to false.

Reloading Unsaved Documents

reload can be called when the document has not yet been persisted. In this case reload performs a find query using the id value specified in the document (and the shard key value, if a shard key is defined):

existing = Band.create!(name: 'Photek')

# Unsaved document
band = Band.new(id: existing.id)
band.reload
band.name
# => "Photek"

Accessing Field Values

Mongoid provides several ways of accessing field values.

Getters & Setters

The recommended way is to use the getter and setter methods generated for each declared field:

class Person
  include Mongoid::Document

  field :first_name
end

person = Person.new

person.first_name = "Artem"
person.first_name
# => "Artem"

To use this mechanism, each field must be explicitly declared, or the model class must enable dynamic fields <dynamic-fields>.

Custom Getters & Setters

It is possible to explicitly define the getter and setter methods to provide custom behavior when reading or writing fields, for example value transformations or storing values under different field names. In this case read_attribute and write_attribute methods can be used to read and write the values directly into the attributes hash:

class Person
  include Mongoid::Document

  def first_name
    read_attribute(:fn)
  end

  def first_name=(value)
    write_attribute(:fn, value)
  end
end

person = Person.new

person.first_name = "Artem"
person.first_name
# => "Artem"

person.attributes
# => {"_id"=>BSON::ObjectId('606477dc2c97a628cf47075b'), "fn"=>"Artem"}

read_attribute & write_attribute [read-write-attribute]

The read_attribute and write_attribute methods can be used explicitly as well. Note that if a field specifies its storage field name <storage-field-names>, both read_attribute and write_attribute accept either the declared field name or the storage field name for operations:

class Person
  include Mongoid::Document

  field :first_name, as: :fn
  field :last_name, as: :ln
end

person = Person.new(first_name: "Artem")
# => #<Person _id: 60647a522c97a6292c195b4b, first_name(fn): "Artem", last_name(ln): nil>

person.read_attribute(:first_name)
# => "Artem"

person.read_attribute(:fn)
# => "Artem"

person.write_attribute(:last_name, "Pushkin")
person
# => #<Person _id: 60647a522c97a6292c195b4b, first_name(fn): "Artem", last_name(ln): "Pushkin">

person.write_attribute(:ln, "Medvedev")
person
# => #<Person _id: 60647a522c97a6292c195b4b, first_name(fn): "Artem", last_name(ln): "Medvedev">

read_attribute and write_attribute do not require that a field with the used name is defined, but writing field values with write_attribute does not cause the respective field to be defined either:

person.write_attribute(:undefined, "Hello")
person
# => #<Person _id: 60647b212c97a6292c195b4c, first_name(fn): "Artem", last_name(ln): "Medvedev">
person.attributes
# => {"_id"=>BSON::ObjectId('60647b212c97a6292c195b4c'), "first_name"=>"Artem", "last_name"=>"Medvedev", "undefined"=>"Hello"}

person.read_attribute(:undefined)
# => "Hello"
person.undefined
# raises NoMethodError

When read_attribute is used to access a missing field, it returns nil.

Hash Access

Mongoid model instances define the [] and []= methods to provide Hash style access to the attributes. [] is an alias for read_attribute and []= is an alias for write_attribute; see the section on read_attribute and write_attribute <read-write-attribute> for the detailed description of their behavior.

class Person
  include Mongoid::Document

  field :first_name, as: :fn
  field :last_name, as: :ln
end

person = Person.new(first_name: "Artem")

person["fn"]
# => "Artem"

person[:first_name]
# => "Artem"

person[:ln] = "Medvedev"
person
# => #<Person _id: 606483742c97a629bdde5cfc, first_name(fn): "Artem", last_name(ln): "Medvedev">

person["last_name"] = "Pushkin"
person
# => #<Person _id: 606483742c97a629bdde5cfc, first_name(fn): "Artem", last_name(ln): "Pushkin">

Bulk Attribute Writes

In cases where you want to set multiple field values at once, there are a few different ways of accomplishing this as well.

# Get the field values as a hash.
person.attributes

# Set the field values in the document.
Person.new(first_name: "Jean-Baptiste", middle_name: "Emmanuel")
person.attributes = { first_name: "Jean-Baptiste", middle_name: "Emmanuel" }
person.write_attributes(
  first_name: "Jean-Baptiste",
  middle_name: "Emmanuel",
)

Dirty Tracking

Mongoid supports tracking of changed or "dirty" fields with an API that mirrors that of Active Model. If a defined field has been modified in a model the model will be marked as dirty and some additional behavior comes into play.

Viewing Changes

There are various ways to view what has been altered on a model. Changes are recorded from the time a document is instantiated, either as a new document or via loading from the database up to the time it is saved. Any persistence operation clears the changes.

class Person
  include Mongoid::Document
  field :name, type: String
end

person = Person.first
person.name = "Alan Garner"

# Check to see if the document has changed.
person.changed? # true

# Get an array of the names of the changed fields.
person.changed # [ :name ]

# Get a hash of the old and changed values for each field.
person.changes # { "name" => [ "Alan Parsons", "Alan Garner" ] }

# Check if a specific field has changed.
person.name_changed? # true

# Get the changes for a specific field.
person.name_change # [ "Alan Parsons", "Alan Garner" ]

# Get the previous value for a field.
person.name_was # "Alan Parsons"

Resetting Changes

You can reset changes of a field to its previous value by calling the reset method.

person = Person.first
person.name = "Alan Garner"

# Reset the changed name back to the original
person.reset_name!
person.name # "Alan Parsons"

Persistence

Mongoid uses dirty tracking as the core of its persistence operations. It looks at the changes on a document and atomically updates only what has changed, unlike other frameworks that write the entire document on each save. If no changes have been made, Mongoid will not hit the database on a call to Model#save.

Viewing Previous Changes

After a document has been persisted, you can see what the changes were previously by calling Model#previous_changes.

person = Person.first
person.name = "Alan Garner"
person.save # Clears out current changes.

# View the previous changes.
person.previous_changes # { "name" => [ "Alan Parsons", "Alan Garner" ] }

Updating Container Fields

Be aware that, until MONGOID-2951 is resolved, all fields including container ones must be assigned to for their values to be persisted to the database.

For example, adding to a set like this does not work:

class Band
  include Mongoid::Document

  field :tours, type: Set
end

band = Band.new
band.tours
# => #<Set: {}>

band.tours << 'London'
# => #<Set: {"London"}>
band.tours
# => #<Set: {}>

Instead, the field value must be modified outside of the model and assigned back to the model as follows:

class Band
  include Mongoid::Document

  field :tours, type: Set
end

band = Band.new

tours = band.tours
# => #<Set: {}>

tours << 'London'
# => #<Set: {"London"}>

band.tours = tours
# => #<Set: {"London"}>

band.tours
# => #<Set: {"London"}>

Readonly Documents

Documents can be marked read-only in two ways, depending on the value of the Mongoid.legacy_readonly feature flag:

If this flag is turned off, a document is marked read-only when the #readonly! method is called on that documnet. A read-only document, with this flag turned off, will raise a ReadonlyDocument error on attempting to perform any persistence operation, including (but not limited to) saving, updating, deleting and destroying. Note that reloading does not reset the read-only state.

band = Band.first
band.readonly? # => false
band.readonly!
band.readonly? # => true
band.name = "The Rolling Stones"
band.save # => raises ReadonlyDocument error
band.reload.readonly? # => true

If this flag is turned on, a document is marked read-only when that document has been projected (i.e. using #only or #without). A read-only document, with this flag turned on, will not be deletable or destroyable (a ReadonlyDocument error will be raised), but will be saveable and updatable. The read-only status is reset on reloading the document.

class Band
  include Mongoid::Document
  field :name, type: String
  field :genre, type: String
end

band = Band.only(:name).first
band.readonly? # => true
band.destroy # => raises ReadonlyDocument error
band.reload.readonly? # => false

Overriding readonly?

Another way to make a document read-only is by overriding the readonly? method:

class Band
  include Mongoid::Document
  field :name, type: String
  field :genre, type: String

  def readonly?
    true
  end
end

band = Band.first
band.readonly? # => true
band.destroy # => raises ReadonlyDocument error