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module ActiveRecord
# = Active Record Identity Map
#
# 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
#
# == Configuration
#
# In order to enable IdentityMap, set <tt>config.active_record.identity_map = true</tt>
# in your <tt>config/application.rb</tt> file.
#
# IdentityMap is disabled by default and still in development (i.e. use it with care).
#
# == Associations
#
# 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.
#
module IdentityMap
class << self
def enabled=(flag)
Thread.current[:identity_map_enabled] = flag
end
def enabled
Thread.current[:identity_map_enabled]
end
alias enabled? enabled
def repository
Thread.current[:identity_map] ||= Hash.new { |h,k| h[k] = {} }
end
def use
old, self.enabled = enabled, true
yield if block_given?
ensure
self.enabled = old
clear
end
def without
old, self.enabled = enabled, false
yield if block_given?
ensure
self.enabled = old
end
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
def add(record)
repository[record.class.symbolized_sti_name][record.id] = record
end
def remove(record)
repository[record.class.symbolized_sti_name].delete(record.id)
end
def remove_by_id(symbolized_sti_name, id)
repository[symbolized_sti_name].delete(id)
end
def clear
repository.clear
end
end
# Reinitialize an Identity Map model object from +coder+.
# +coder+ must contain the attributes necessary for initializing an empty
# model object.
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
class Middleware
def initialize(app)
@app = app
end
def call(env)
enabled = IdentityMap.enabled
IdentityMap.enabled = true
response = @app.call(env)
response[2] = Rack::BodyProxy.new(response[2]) do
IdentityMap.enabled = enabled
IdentityMap.clear
end
response
end
end
end
end
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