1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
|
# frozen_string_literal: true
module ActiveRecord
module Associations
# Implements the details of eager loading of Active Record associations.
#
# Suppose that you have the following two Active Record models:
#
# class Author < ActiveRecord::Base
# # columns: name, age
# has_many :books
# end
#
# class Book < ActiveRecord::Base
# # columns: title, sales, author_id
# end
#
# When you load an author with all associated books Active Record will make
# multiple queries like this:
#
# Author.includes(:books).where(name: ['bell hooks', 'Homer']).to_a
#
# => SELECT `authors`.* FROM `authors` WHERE `name` IN ('bell hooks', 'Homer')
# => SELECT `books`.* FROM `books` WHERE `author_id` IN (2, 5)
#
# Active Record saves the ids of the records from the first query to use in
# the second. Depending on the number of associations involved there can be
# arbitrarily many SQL queries made.
#
# However, if there is a WHERE clause that spans across tables Active
# Record will fall back to a slightly more resource-intensive single query:
#
# Author.includes(:books).where(books: {title: 'Illiad'}).to_a
# => SELECT `authors`.`id` AS t0_r0, `authors`.`name` AS t0_r1, `authors`.`age` AS t0_r2,
# `books`.`id` AS t1_r0, `books`.`title` AS t1_r1, `books`.`sales` AS t1_r2
# FROM `authors`
# LEFT OUTER JOIN `books` ON `authors`.`id` = `books`.`author_id`
# WHERE `books`.`title` = 'Illiad'
#
# This could result in many rows that contain redundant data and it performs poorly at scale
# and is therefore only used when necessary.
#
class Preloader #:nodoc:
extend ActiveSupport::Autoload
eager_autoload do
autoload :Association, "active_record/associations/preloader/association"
autoload :ThroughAssociation, "active_record/associations/preloader/through_association"
end
# 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
# <tt>belongs_to :author</tt>, <tt>has_many :buyers</tt> 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 <tt>[:avatar, :books]</tt>
# 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 <tt>{ author: :avatar }</tt> will preload a
# book's author, as well as that author's avatar.
#
# +:associations+ has the same format as the +:include+ option for
# <tt>ActiveRecord::Base.find</tt>. So +associations+ could look like this:
#
# :books
# [ :books, :author ]
# { author: :avatar }
# [ :books, { author: :avatar } ]
def preload(records, associations, preload_scope = nil)
records = Array.wrap(records).compact
if records.empty?
[]
else
Array.wrap(associations).flat_map { |association|
preloaders_on association, records, preload_scope
}
end
end
private
# Loads all the given data into +records+ for the +association+.
def preloaders_on(association, records, scope, polymorphic_parent = false)
case association
when Hash
preloaders_for_hash(association, records, scope, polymorphic_parent)
when Symbol, String
preloaders_for_one(association, records, scope, polymorphic_parent)
else
raise ArgumentError, "#{association.inspect} was not recognized for preload"
end
end
def preloaders_for_hash(association, records, scope, polymorphic_parent)
association.flat_map { |parent, child|
grouped_records(parent, records, polymorphic_parent).flat_map do |reflection, reflection_records|
loaders = preloaders_for_reflection(reflection, reflection_records, scope)
recs = loaders.flat_map(&:preloaded_records)
child_polymorphic_parent = reflection && reflection.options[:polymorphic]
loaders.concat Array.wrap(child).flat_map { |assoc|
preloaders_on assoc, recs, scope, child_polymorphic_parent
}
loaders
end
}
end
# Loads all the given data into +records+ for a singular +association+.
#
# Functions by instantiating a preloader class such as Preloader::Association and
# call the +run+ method for each passed in class in the +records+ argument.
#
# 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
#
# Additionally, polymorphic belongs_to associations can have multiple associated
# classes, depending on the polymorphic_type field. So we group by the classes as
# well.
def preloaders_for_one(association, records, scope, polymorphic_parent)
grouped_records(association, records, polymorphic_parent)
.flat_map do |reflection, reflection_records|
preloaders_for_reflection reflection, reflection_records, scope
end
end
def preloaders_for_reflection(reflection, records, scope)
records.group_by { |record| record.association(reflection.name).klass }.map do |rhs_klass, rs|
loader = preloader_for(reflection, rs).new(rhs_klass, rs, reflection, scope)
loader.run self
loader
end
end
def grouped_records(association, records, polymorphic_parent)
h = {}
records.each do |record|
next unless record
reflection = record.class._reflect_on_association(association)
next if polymorphic_parent && !reflection || !record.association(association).klass
(h[reflection] ||= []) << record
end
h
end
class AlreadyLoaded # :nodoc:
def initialize(klass, owners, reflection, preload_scope)
@owners = owners
@reflection = reflection
end
def run(preloader); end
def preloaded_records
owners.flat_map { |owner| owner.association(reflection.name).target }
end
private
attr_reader :owners, :reflection
end
# Returns a class containing the logic needed to load preload the data
# and attach it to a relation. The class returned implements a `run` method
# that accepts a preloader.
def preloader_for(reflection, owners)
if owners.first.association(reflection.name).loaded?
return AlreadyLoaded
end
reflection.check_preloadable!
if reflection.options[:through]
ThroughAssociation
else
Association
end
end
end
end
end
|
