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de6b4efa3e
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>
194 lines
5.3 KiB
Ruby
194 lines
5.3 KiB
Ruby
# frozen_string_literal: true
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#--
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# Copyright (c) 2004-2021 David Heinemeier Hansson
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#
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# Permission is hereby granted, free of charge, to any person obtaining
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# a copy of this software and associated documentation files (the
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# "Software"), to deal in the Software without restriction, including
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# without limitation the rights to use, copy, modify, merge, publish,
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# distribute, sublicense, and/or sell copies of the Software, and to
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# permit persons to whom the Software is furnished to do so, subject to
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# the following conditions:
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#
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# The above copyright notice and this permission notice shall be
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# included in all copies or substantial portions of the Software.
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#
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# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
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# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
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# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
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# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
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# LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
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# OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
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# WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
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#++
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require "active_support"
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require "active_support/rails"
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require "active_model"
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require "arel"
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require "yaml"
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require "active_record/version"
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require "active_model/attribute_set"
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require "active_record/errors"
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module ActiveRecord
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extend ActiveSupport::Autoload
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autoload :Base
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autoload :Callbacks
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autoload :Core
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autoload :ConnectionHandling
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autoload :CounterCache
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autoload :DynamicMatchers
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autoload :DelegatedType
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autoload :Encryption
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autoload :Enum
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autoload :InternalMetadata
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autoload :Explain
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autoload :Inheritance
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autoload :Integration
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autoload :Migration
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autoload :Migrator, "active_record/migration"
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autoload :ModelSchema
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autoload :NestedAttributes
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autoload :NoTouching
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autoload :TouchLater
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autoload :Persistence
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autoload :QueryCache
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autoload :Querying
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autoload :ReadonlyAttributes
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autoload :RecordInvalid, "active_record/validations"
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autoload :Reflection
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autoload :RuntimeRegistry
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autoload :Sanitization
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autoload :Schema
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autoload :SchemaDumper
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autoload :SchemaMigration
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autoload :Scoping
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autoload :Serialization
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autoload :Store
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autoload :SignedId
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autoload :Suppressor
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autoload :Timestamp
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autoload :Transactions
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autoload :Translation
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autoload :Validations
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autoload :SecureToken
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autoload :DestroyAssociationAsyncJob
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eager_autoload do
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autoload :StatementCache
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autoload :ConnectionAdapters
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autoload :Aggregations
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autoload :Associations
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autoload :AttributeAssignment
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autoload :AttributeMethods
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autoload :AutosaveAssociation
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autoload :AsynchronousQueriesTracker
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autoload :LegacyYamlAdapter
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autoload :Relation
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autoload :AssociationRelation
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autoload :DisableJoinsAssociationRelation
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autoload :NullRelation
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autoload_under "relation" do
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autoload :QueryMethods
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autoload :FinderMethods
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autoload :Calculations
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autoload :PredicateBuilder
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autoload :SpawnMethods
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autoload :Batches
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autoload :Delegation
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end
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autoload :Result
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autoload :FutureResult
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autoload :TableMetadata
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autoload :Type
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end
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module Coders
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autoload :YAMLColumn, "active_record/coders/yaml_column"
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autoload :JSON, "active_record/coders/json"
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end
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module AttributeMethods
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extend ActiveSupport::Autoload
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eager_autoload do
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autoload :BeforeTypeCast
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autoload :Dirty
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autoload :PrimaryKey
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autoload :Query
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autoload :Read
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autoload :TimeZoneConversion
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autoload :Write
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autoload :Serialization
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end
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end
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module Locking
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extend ActiveSupport::Autoload
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eager_autoload do
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autoload :Optimistic
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autoload :Pessimistic
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end
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end
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module Scoping
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extend ActiveSupport::Autoload
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eager_autoload do
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autoload :Named
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autoload :Default
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end
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end
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module Middleware
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extend ActiveSupport::Autoload
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autoload :DatabaseSelector, "active_record/middleware/database_selector"
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end
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module Tasks
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extend ActiveSupport::Autoload
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autoload :DatabaseTasks
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autoload :SQLiteDatabaseTasks, "active_record/tasks/sqlite_database_tasks"
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autoload :MySQLDatabaseTasks, "active_record/tasks/mysql_database_tasks"
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autoload :PostgreSQLDatabaseTasks,
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"active_record/tasks/postgresql_database_tasks"
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end
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autoload :TestDatabases, "active_record/test_databases"
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autoload :TestFixtures, "active_record/fixtures"
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def self.eager_load!
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super
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ActiveRecord::Locking.eager_load!
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ActiveRecord::Scoping.eager_load!
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ActiveRecord::Associations.eager_load!
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ActiveRecord::AttributeMethods.eager_load!
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ActiveRecord::ConnectionAdapters.eager_load!
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ActiveRecord::Encryption.eager_load!
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end
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end
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ActiveSupport.on_load(:active_record) do
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Arel::Table.engine = self
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end
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ActiveSupport.on_load(:i18n) do
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I18n.load_path << File.expand_path("active_record/locale/en.yml", __dir__)
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end
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YAML.load_tags["!ruby/object:ActiveRecord::AttributeSet"] = "ActiveModel::AttributeSet"
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YAML.load_tags["!ruby/object:ActiveRecord::Attribute::FromDatabase"] = "ActiveModel::Attribute::FromDatabase"
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YAML.load_tags["!ruby/object:ActiveRecord::LazyAttributeHash"] = "ActiveModel::LazyAttributeHash"
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YAML.load_tags["!ruby/object:ActiveRecord::ConnectionAdapters::AbstractMysqlAdapter::MysqlString"] = "ActiveRecord::Type::String"
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