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rails--rails/activerecord/lib/active_record.rb
eileencodes de6b4efa3e
Add option to skip joins for associations.
In a multiple database application, associations can't join across
databases. When set, this option tells Rails to make 2 or more queries
rather than using joins for associations.

Set the option on a has many through association:

```ruby
class Dog
  has_many :treats, through: :humans, disable_joins: true
  has_many :humans
end
```

Then instead of generating join SQL, two queries are used for `@dog.treats`:

```
SELECT "humans"."id" FROM "humans" WHERE "humans"."dog_id" = ?  [["dog_id", 1]]
SELECT "treats".* FROM "treats" WHERE "treats"."human_id" IN (?, ?, ?)  [["human_id", 1], ["human_id", 2], ["human_id", 3]]
```

This code is extracted from a gem we use internally at GitHub which
means the implementation here is used in production daily and isn't
experimental.

I often get the question "why can't Rails do this automatically" so I
figured I'd include the answer in the commit. Rails can't do this
automatically because associations are lazily loaded. `dog.treats` needs
to load `Dog`, then `Human` and then `Treats`. When `dog.treats` is
called Rails pre-generates the SQL that will be run and puts that
information into a reflection object. Because the SQL parts are pre-generated,
as soon as `dog.treats` is loaded it's too late to skip a join. The join
is already available on the object and that join is what's run to load
`treats` from `dog` through `humans`. I think the only way to avoid setting
an option on the association is to rewrite how and when the SQL is
generated for associations which is a large undertaking. Basically the
way that Active Record associations are designed, it is currently
impossible to have Rails figure out to not join (loading the association
will cause the join to occur, and that join will raise an error if the
models don't live in the same db).

The original implementation was written by me and Aaron. Lee helped port
over tests, and I refactored the extraction to better match Rails style.

Co-authored-by: Lee Quarella <leequarella@gmail.com>
Co-authored-by: Aaron Patterson <aaron@rubyonrails.org>
2021-04-19 11:17:31 -04:00

194 lines
5.3 KiB
Ruby

# frozen_string_literal: true
#--
# Copyright (c) 2004-2021 David Heinemeier Hansson
#
# Permission is hereby granted, free of charge, to any person obtaining
# a copy of this software and associated documentation files (the
# "Software"), to deal in the Software without restriction, including
# without limitation the rights to use, copy, modify, merge, publish,
# distribute, sublicense, and/or sell copies of the Software, and to
# permit persons to whom the Software is furnished to do so, subject to
# the following conditions:
#
# The above copyright notice and this permission notice shall be
# included in all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
# LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
# OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
# WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
#++
require "active_support"
require "active_support/rails"
require "active_model"
require "arel"
require "yaml"
require "active_record/version"
require "active_model/attribute_set"
require "active_record/errors"
module ActiveRecord
extend ActiveSupport::Autoload
autoload :Base
autoload :Callbacks
autoload :Core
autoload :ConnectionHandling
autoload :CounterCache
autoload :DynamicMatchers
autoload :DelegatedType
autoload :Encryption
autoload :Enum
autoload :InternalMetadata
autoload :Explain
autoload :Inheritance
autoload :Integration
autoload :Migration
autoload :Migrator, "active_record/migration"
autoload :ModelSchema
autoload :NestedAttributes
autoload :NoTouching
autoload :TouchLater
autoload :Persistence
autoload :QueryCache
autoload :Querying
autoload :ReadonlyAttributes
autoload :RecordInvalid, "active_record/validations"
autoload :Reflection
autoload :RuntimeRegistry
autoload :Sanitization
autoload :Schema
autoload :SchemaDumper
autoload :SchemaMigration
autoload :Scoping
autoload :Serialization
autoload :Store
autoload :SignedId
autoload :Suppressor
autoload :Timestamp
autoload :Transactions
autoload :Translation
autoload :Validations
autoload :SecureToken
autoload :DestroyAssociationAsyncJob
eager_autoload do
autoload :StatementCache
autoload :ConnectionAdapters
autoload :Aggregations
autoload :Associations
autoload :AttributeAssignment
autoload :AttributeMethods
autoload :AutosaveAssociation
autoload :AsynchronousQueriesTracker
autoload :LegacyYamlAdapter
autoload :Relation
autoload :AssociationRelation
autoload :DisableJoinsAssociationRelation
autoload :NullRelation
autoload_under "relation" do
autoload :QueryMethods
autoload :FinderMethods
autoload :Calculations
autoload :PredicateBuilder
autoload :SpawnMethods
autoload :Batches
autoload :Delegation
end
autoload :Result
autoload :FutureResult
autoload :TableMetadata
autoload :Type
end
module Coders
autoload :YAMLColumn, "active_record/coders/yaml_column"
autoload :JSON, "active_record/coders/json"
end
module AttributeMethods
extend ActiveSupport::Autoload
eager_autoload do
autoload :BeforeTypeCast
autoload :Dirty
autoload :PrimaryKey
autoload :Query
autoload :Read
autoload :TimeZoneConversion
autoload :Write
autoload :Serialization
end
end
module Locking
extend ActiveSupport::Autoload
eager_autoload do
autoload :Optimistic
autoload :Pessimistic
end
end
module Scoping
extend ActiveSupport::Autoload
eager_autoload do
autoload :Named
autoload :Default
end
end
module Middleware
extend ActiveSupport::Autoload
autoload :DatabaseSelector, "active_record/middleware/database_selector"
end
module Tasks
extend ActiveSupport::Autoload
autoload :DatabaseTasks
autoload :SQLiteDatabaseTasks, "active_record/tasks/sqlite_database_tasks"
autoload :MySQLDatabaseTasks, "active_record/tasks/mysql_database_tasks"
autoload :PostgreSQLDatabaseTasks,
"active_record/tasks/postgresql_database_tasks"
end
autoload :TestDatabases, "active_record/test_databases"
autoload :TestFixtures, "active_record/fixtures"
def self.eager_load!
super
ActiveRecord::Locking.eager_load!
ActiveRecord::Scoping.eager_load!
ActiveRecord::Associations.eager_load!
ActiveRecord::AttributeMethods.eager_load!
ActiveRecord::ConnectionAdapters.eager_load!
ActiveRecord::Encryption.eager_load!
end
end
ActiveSupport.on_load(:active_record) do
Arel::Table.engine = self
end
ActiveSupport.on_load(:i18n) do
I18n.load_path << File.expand_path("active_record/locale/en.yml", __dir__)
end
YAML.load_tags["!ruby/object:ActiveRecord::AttributeSet"] = "ActiveModel::AttributeSet"
YAML.load_tags["!ruby/object:ActiveRecord::Attribute::FromDatabase"] = "ActiveModel::Attribute::FromDatabase"
YAML.load_tags["!ruby/object:ActiveRecord::LazyAttributeHash"] = "ActiveModel::LazyAttributeHash"
YAML.load_tags["!ruby/object:ActiveRecord::ConnectionAdapters::AbstractMysqlAdapter::MysqlString"] = "ActiveRecord::Type::String"