Ensures that each object gets loaded only once by keeping every loaded object in a map. Looks up objects using the map when referring to them.
More information on Identity Map pattern:
http://www.martinfowler.com/eaaCatalog/identityMap.html
In order to enable IdentityMap, set config.active_record.identity_map = true in your config/application.rb file.
IdentityMap is disabled by default and still in development (i.e. use it with care).
Active Record Identity Map does not track associations yet. For example:
comment = @post.comments.first comment.post = nil @post.comments.include?(comment) #=> true
Ideally, the example above would return false, removing the comment object from the post association when the association is nullified. This may cause side effects, as in the situation below, if Identity Map is enabled:
Post.has_many :comments, :dependent => :destroy comment = @post.comments.first comment.post = nil comment.save Post.destroy(@post.id)
Without using Identity Map, the code above will destroy the @post object leaving the comment object intact. However, once we enable Identity Map, the post loaded by Post.destroy is exactly the same object as the object @post. As the object @post still has the comment object in @post.comments, once Identity Map is enabled, the comment object will be accidently removed.
This inconsistency is meant to be fixed in future Rails releases.
# File lib/active_record/identity_map.rb, line 92 def add(record) repository[record.class.symbolized_sti_name][record.id] = record if contain_all_columns?(record) end
# File lib/active_record/identity_map.rb, line 104 def clear repository.clear end
# File lib/active_record/identity_map.rb, line 51 def enabled Thread.current[:identity_map_enabled] end
# File lib/active_record/identity_map.rb, line 47 def enabled=(flag) Thread.current[:identity_map_enabled] = flag end
# File lib/active_record/identity_map.rb, line 77 def get(klass, primary_key) record = repository[klass.symbolized_sti_name][primary_key] if record.is_a?(klass) ActiveSupport::Notifications.instrument("identity.active_record", :line => "From Identity Map (id: #{primary_key})", :name => "#{klass} Loaded", :connection_id => object_id) record else nil end end
# File lib/active_record/identity_map.rb, line 96 def remove(record) repository[record.class.symbolized_sti_name].delete(record.id) end
# File lib/active_record/identity_map.rb, line 100 def remove_by_id(symbolized_sti_name, id) repository[symbolized_sti_name].delete(id) end
# File lib/active_record/identity_map.rb, line 56 def repository Thread.current[:identity_map] ||= Hash.new { |h,k| h[k] = {} } end
Reinitialize an Identity Map model object from coder. coder must contain the attributes necessary for initializing an empty model object.
# File lib/active_record/identity_map.rb, line 118 def reinit_with(coder) @attributes_cache = {} dirty = @changed_attributes.keys attributes = self.class.initialize_attributes(coder['attributes'].except(*dirty)) @attributes.update(attributes) @changed_attributes.update(coder['attributes'].slice(*dirty)) @changed_attributes.delete_if{|k,v| v.eql? @attributes[k]} run_callbacks :find self end
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