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
|
module ActiveRecord
module Associations
# Implements the details of eager loading of Active Record associations.
#
# 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.author_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.
class Preloader #:nodoc:
extend ActiveSupport::Autoload
eager_autoload do
autoload :Association, 'active_record/associations/preloader/association'
autoload :SingularAssociation, 'active_record/associations/preloader/singular_association'
autoload :CollectionAssociation, 'active_record/associations/preloader/collection_association'
autoload :ThroughAssociation, 'active_record/associations/preloader/through_association'
autoload :HasMany, 'active_record/associations/preloader/has_many'
autoload :HasManyThrough, 'active_record/associations/preloader/has_many_through'
autoload :HasOne, 'active_record/associations/preloader/has_one'
autoload :HasOneThrough, 'active_record/associations/preloader/has_one_through'
autoload :BelongsTo, 'active_record/associations/preloader/belongs_to'
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 } ]
NULL_RELATION = Struct.new(:values, :bind_values).new({}, [])
def preload(records, associations, preload_scope = nil)
records = Array.wrap(records).compact.uniq
associations = Array.wrap(associations)
preload_scope = preload_scope || NULL_RELATION
if records.empty?
[]
else
associations.flat_map { |association|
preloaders_on association, records, preload_scope
}
end
end
private
def preloaders_on(association, records, scope)
case association
when Hash
preloaders_for_hash(association, records, scope)
when Symbol
preloaders_for_one(association, records, scope)
when String
preloaders_for_one(association.to_sym, records, scope)
else
raise ArgumentError, "#{association.inspect} was not recognised for preload"
end
end
def preloaders_for_hash(association, records, scope)
parent, child = association.to_a.first # hash should only be of length 1
loaders = preloaders_for_one parent, records, scope
recs = loaders.flat_map(&:preloaded_records).uniq
loaders.concat Array.wrap(child).flat_map { |assoc|
preloaders_on assoc, recs, scope
}
end
# 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)
grouped_records(association, records).flat_map do |reflection, klasses|
klasses.map do |rhs_klass, rs|
loader = preloader_for(reflection, rs, rhs_klass).new(rhs_klass, rs, reflection, scope)
loader.run self
loader
end
end
end
def grouped_records(association, records)
h = {}
records.each do |record|
assoc = record.association(association)
klasses = h[assoc.reflection] ||= {}
(klasses[assoc.klass] ||= []) << record
end
h
end
class AlreadyLoaded
attr_reader :owners, :reflection
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
end
class NullPreloader
def self.new(klass, owners, reflection, preload_scope); self; end
def self.run(preloader); end
end
def preloader_for(reflection, owners, rhs_klass)
return NullPreloader unless rhs_klass
if owners.first.association(reflection.name).loaded?
return AlreadyLoaded
end
reflection.check_preloadable!
case reflection.macro
when :has_many
reflection.options[:through] ? HasManyThrough : HasMany
when :has_one
reflection.options[:through] ? HasOneThrough : HasOne
when :belongs_to
BelongsTo
end
end
end
end
end
|
