require 'active_support/core_ext/array/wrap'
require 'active_support/core_ext/enumerable'
module ActiveRecord
# See ActiveRecord::AssociationPreload::ClassMethods for documentation.
module AssociationPreload #:nodoc:
extend ActiveSupport::Concern
# Implements the details of eager loading of Active Record associations.
# Application developers should not use this module directly.
#
# ActiveRecord::Base is extended with this module. The source code in
# ActiveRecord::Base references methods defined in this module.
#
# Note that 'eager loading' and 'preloading' are actually the same thing.
# However, there are two different eager loading strategies.
#
# The first one is by using table joins. This was only strategy available
# prior to Rails 2.1. Suppose that you have an Author model with columns
# 'name' and 'age', and a Book model with columns 'name' and 'sales'. Using
# this strategy, Active Record would try to retrieve all data for an author
# and all of its books via a single query:
#
# SELECT * FROM authors
# LEFT OUTER JOIN books ON authors.id = books.id
# WHERE authors.name = 'Ken Akamatsu'
#
# However, this could result in many rows that contain redundant data. After
# having received the first row, we already have enough data to instantiate
# the Author object. In all subsequent rows, only the data for the joined
# 'books' table is useful; the joined 'authors' data is just redundant, and
# processing this redundant data takes memory and CPU time. The problem
# quickly becomes worse and worse as the level of eager loading increases
# (i.e. if Active Record is to eager load the associations' associations as
# well).
#
# The second strategy is to use multiple database queries, one for each
# level of association. Since Rails 2.1, this is the default strategy. In
# situations where a table join is necessary (e.g. when the +:conditions+
# option references an association's column), it will fallback to the table
# join strategy.
#
# See also ActiveRecord::Associations::ClassMethods, which explains eager
# loading in a more high-level (application developer-friendly) manner.
module ClassMethods
protected
# Eager loads the named associations for the given Active Record record(s).
#
# In this description, 'association name' shall refer to the name passed
# to an association creation method. For example, a model that specifies
# belongs_to :author, has_many :buyers has association
# names +:author+ and +:buyers+.
#
# == Parameters
# +records+ is an array of ActiveRecord::Base. This array needs not be flat,
# i.e. +records+ itself may also contain arrays of records. In any case,
# +preload_associations+ will preload the all associations records by
# flattening +records+.
#
# +associations+ specifies one or more associations that you want to
# preload. It may be:
# - a Symbol or a String which specifies a single association name. For
# example, specifying +:books+ allows this method to preload all books
# for an Author.
# - an Array which specifies multiple association names. This array
# is processed recursively. For example, specifying [:avatar, :books]
# allows this method to preload an author's avatar as well as all of his
# books.
# - a Hash which specifies multiple association names, as well as
# association names for the to-be-preloaded association objects. For
# example, specifying { :author => :avatar } will preload a
# book's author, as well as that author's avatar.
#
# +:associations+ has the same format as the +:include+ option for
# ActiveRecord::Base.find. So +associations+ could look like this:
#
# :books
# [ :books, :author ]
# { :author => :avatar }
# [ :books, { :author => :avatar } ]
#
# +preload_options+ contains options that will be passed to ActiveRecord::Base#find
# (which is called under the hood for preloading records). But it is passed
# only one level deep in the +associations+ argument, i.e. it's not passed
# to the child associations when +associations+ is a Hash.
def preload_associations(records, associations, preload_options={})
records = Array.wrap(records).compact.uniq
return if records.empty?
case associations
when Array then associations.each {|association| preload_associations(records, association, preload_options)}
when Symbol, String then preload_one_association(records, associations.to_sym, preload_options)
when Hash then
associations.each do |parent, child|
raise "parent must be an association name" unless parent.is_a?(String) || parent.is_a?(Symbol)
preload_associations(records, parent, preload_options)
reflection = reflections[parent]
parents = records.sum { |record| Array.wrap(record.send(reflection.name)) }
unless parents.empty?
parents.first.class.preload_associations(parents, child)
end
end
end
end
private
# Preloads a specific named association for the given records. This is
# called by +preload_associations+ as its base case.
def preload_one_association(records, association, preload_options={})
class_to_reflection = {}
# Not all records have the same class, so group then preload
# group on the reflection itself so that if various subclass share the same association then
# we do not split them unnecessarily
records.group_by { |record| class_to_reflection[record.class] ||= record.class.reflections[association]}.each do |reflection, _records|
raise ConfigurationError, "Association named '#{ association }' was not found; perhaps you misspelled it?" unless reflection
# 'reflection.macro' can return 'belongs_to', 'has_many', etc. Thus,
# the following could call 'preload_belongs_to_association',
# 'preload_has_many_association', etc.
send("preload_#{reflection.macro}_association", _records, reflection, preload_options)
end
end
def add_preloaded_records_to_collection(parent_records, reflection_name, associated_record)
parent_records.each do |parent_record|
association_proxy = parent_record.send(reflection_name)
association_proxy.loaded
association_proxy.target.concat(Array.wrap(associated_record))
association_proxy.send(:set_inverse_instance, associated_record)
end
end
def add_preloaded_record_to_collection(parent_records, reflection_name, associated_record)
parent_records.each do |parent_record|
parent_record.send(:association_proxy, reflection_name).target = associated_record
end
end
def set_association_collection_records(id_to_parent_map, reflection_name, associated_records, key)
associated_records.each do |associated_record|
parent_records = id_to_parent_map[associated_record[key].to_s]
add_preloaded_records_to_collection(parent_records, reflection_name, associated_record)
end
end
def set_association_single_records(id_to_record_map, reflection_name, associated_records, key)
seen_keys = {}
associated_records.each do |associated_record|
seen_key = associated_record[key].to_s
#this is a has_one or belongs_to: there should only be one record.
#Unfortunately we can't (in portable way) ask the database for
#'all records where foo_id in (x,y,z), but please
# only one row per distinct foo_id' so this where we enforce that
next if seen_keys.key? seen_key
seen_keys[seen_key] = true
mapped_records = id_to_record_map[seen_key]
mapped_records.each do |mapped_record|
association_proxy = mapped_record.send(:association_proxy, reflection_name)
association_proxy.target = associated_record
association_proxy.send(:set_inverse_instance, associated_record)
end
end
id_to_record_map.each do |id, records|
next if seen_keys.include?(id)
add_preloaded_record_to_collection(records, reflection_name, nil)
end
end
# Given a collection of Active Record objects, constructs a Hash which maps
# the objects' IDs to the relevant objects. Returns a 2-tuple
# (id_to_record_map, ids) where +id_to_record_map+ is the Hash,
# and +ids+ is an Array of record IDs.
def construct_id_map(records, primary_key=nil)
records.group_by do |record|
primary_key ||= record.class.primary_key
record[primary_key].to_s
end
end
def preload_has_and_belongs_to_many_association(records, reflection, preload_options={})
left = reflection.klass.arel_table
id_to_record_map = construct_id_map(records)
records.each {|record| record.send(reflection.name).loaded}
options = reflection.options
right = Arel::Table.new(options[:join_table]).alias('t0')
join_condition = left[reflection.klass.primary_key].eq(
right[reflection.association_foreign_key])
join = left.create_join(right, left.create_on(join_condition))
select = [
# FIXME: options[:select] is always nil in the tests. Do we really
# need it?
options[:select] || left[Arel.star],
right[reflection.foreign_key].as(
Arel.sql('the_parent_record_id'))
]
associated_records_proxy = reflection.klass.unscoped.
includes(options[:include]).
order(options[:order])
associated_records_proxy.joins_values = [join]
associated_records_proxy.select_values = select
custom_conditions = append_conditions(reflection, preload_options)
klass = associated_records_proxy.klass
associated_records(id_to_record_map.keys) { |some_ids|
method = in_or_equal(some_ids)
conditions = right.create_and(
[right[reflection.foreign_key].send(*method)] +
custom_conditions)
relation = associated_records_proxy.where(conditions)
klass.connection.select_all(relation.arel.to_sql, 'SQL', relation.bind_values)
}.map! { |row|
parent_records = id_to_record_map[row['the_parent_record_id'].to_s]
associated_record = klass.instantiate row
add_preloaded_records_to_collection(
parent_records, reflection.name, associated_record)
associated_record
}
end
def preload_has_one_association(records, reflection, preload_options={})
return if records.first.send(:association_proxy, reflection.name).loaded?
id_to_record_map = construct_id_map(records, reflection.options[:primary_key])
options = reflection.options
add_preloaded_record_to_collection(records, reflection.name, nil)
if options[:through]
through_records = preload_through_records(records, reflection, options[:through])
unless through_records.empty?
through_reflection = reflections[options[:through]]
through_primary_key = through_reflection.foreign_key
source = reflection.source_reflection.name
through_records.first.class.preload_associations(through_records, source)
if through_reflection.macro == :belongs_to
id_to_record_map = construct_id_map(records, through_primary_key)
through_primary_key = through_reflection.klass.primary_key
end
through_records.each do |through_record|
add_preloaded_record_to_collection(id_to_record_map[through_record[through_primary_key].to_s],
reflection.name, through_record.send(source))
end
end
else
set_association_single_records(id_to_record_map, reflection.name, find_associated_records(id_to_record_map.keys, reflection, preload_options), reflection.foreign_key)
end
end
def preload_has_many_association(records, reflection, preload_options={})
return if records.first.send(reflection.name).loaded?
options = reflection.options
foreign_key = reflection.through_reflection_foreign_key
id_to_record_map = construct_id_map(records, foreign_key || reflection.options[:primary_key])
records.each {|record| record.send(reflection.name).loaded}
if options[:through]
through_records = preload_through_records(records, reflection, options[:through])
unless through_records.empty?
source = reflection.source_reflection.name
through_records.first.class.preload_associations(through_records, source, options)
through_records.each do |through_record|
through_record_id = through_record[reflection.through_reflection_primary_key].to_s
add_preloaded_records_to_collection(id_to_record_map[through_record_id], reflection.name, through_record.send(source))
end
records.each { |record| record.send(reflection.name).target.uniq! } if options[:uniq]
end
else
set_association_collection_records(id_to_record_map, reflection.name, find_associated_records(id_to_record_map.keys, reflection, preload_options),
reflection.foreign_key)
end
end
def preload_through_records(records, reflection, through_association)
if reflection.options[:source_type]
interface = reflection.source_reflection.foreign_type
preload_options = {:conditions => ["#{connection.quote_column_name interface} = ?", reflection.options[:source_type]]}
records.compact!
records.first.class.preload_associations(records, through_association, preload_options)
# Dont cache the association - we would only be caching a subset
records.map { |record|
proxy = record.send(through_association)
if proxy.respond_to?(:target)
Array.wrap(proxy.target).tap { proxy.reset }
else # this is a has_one :through reflection
[proxy].compact
end
}.flatten(1)
else
options = {}
options[:include] = reflection.options[:include] || reflection.options[:source] if reflection.options[:conditions]
options[:order] = reflection.options[:order]
options[:conditions] = reflection.options[:conditions]
records.first.class.preload_associations(records, through_association, options)
records.map { |record|
Array.wrap(record.send(through_association))
}.flatten(1)
end
end
def preload_belongs_to_association(records, reflection, preload_options={})
return if records.first.send(:association_proxy, reflection.name).loaded?
options = reflection.options
klasses_and_ids = {}
if options[:polymorphic]
# Construct a mapping from klass to a list of ids to load and a mapping of those ids back
# to their parent_records
records.each do |record|
if klass = record.send(reflection.foreign_type)
klass_id = record.send(reflection.foreign_key)
if klass_id
id_map = klasses_and_ids[klass.constantize] ||= {}
(id_map[klass_id.to_s] ||= []) << record
end
end
end
else
id_map = records.group_by do |record|
key = record.send(reflection.foreign_key)
key && key.to_s
end
klasses_and_ids[reflection.klass] = id_map unless id_map.empty?
end
klasses_and_ids.each do |klass, _id_map|
primary_key = (reflection.options[:primary_key] || klass.primary_key).to_s
keys = _id_map.keys.compact
unless keys.empty?
table = klass.arel_table
method = in_or_equal(keys)
conditions = table[primary_key].send(*method)
custom_conditions = append_conditions(reflection, preload_options)
conditions = custom_conditions.inject(conditions) do |ast, cond|
ast.and cond
end
associated_records = klass.unscoped.where(conditions).apply_finder_options(options.slice(:include, :select, :joins, :order)).to_a
else
associated_records = []
end
set_association_single_records(_id_map, reflection.name, associated_records, primary_key)
end
end
def find_associated_records(ids, reflection, preload_options)
options = reflection.options
table = reflection.klass.arel_table
conditions = []
key = reflection.foreign_key
if interface = reflection.options[:as]
key = "#{interface}_id"
conditions << table["#{interface}_type"].eq(base_class.sti_name)
end
conditions.concat append_conditions(reflection, preload_options)
find_options = {
:select => preload_options[:select] || options[:select] || table[Arel.star],
:include => preload_options[:include] || options[:include],
:joins => options[:joins],
:group => preload_options[:group] || options[:group],
:order => preload_options[:order] || options[:order]
}
associated_records(ids) do |some_ids|
method = in_or_equal(some_ids)
where = table.create_and(conditions + [table[key].send(*method)])
reflection.klass.scoped.apply_finder_options(find_options.merge(:conditions => where)).to_a
end
end
def append_conditions(reflection, preload_options)
[
("(#{reflection.sanitized_conditions})" if reflection.sanitized_conditions),
("(#{sanitize_sql preload_options[:conditions]})" if preload_options[:conditions]),
].compact.map { |x| Arel.sql x }
end
def in_or_equal(ids)
ids.length == 1 ? ['eq', ids.first] : ['in', ids]
end
# Some databases impose a limit on the number of ids in a list (in Oracle its 1000)
# Make several smaller queries if necessary or make one query if the adapter supports it
def associated_records(ids)
in_clause_length = connection.in_clause_length || ids.size
records = []
ids.each_slice(in_clause_length) do |some_ids|
records.concat yield(some_ids)
end
records
end
end
end
end