Add configurable deprecation warning for singular associations

This removes the singularize from `where` which runs on all `expand_from_hash` keys which might be reflections or column names. This saves a lot of time by avoiding singularizing column names.

Previously in #45163 the singularize was removed entirely. after some reflection, I think it is better to at least give a warning for one release since `where` is a very popular API and the problems you can run into with incorrect relation could be hard to debug.

Configurable with `ActiveRecord::Base.allow_deprecated_singular_assocaitions_name = false` / `config.active_record.allow_deprecated_singular_assocaitions_name = false`

activerecord/CHANGELOG.md

@@ -1,3 +1,10 @@
+*   Add configurable deprecation warning for singular associations
+
+    This adds a deprecation warning when using the plural name of a singular associations in `where`.
+    It is possible to opt into the new more performant behavior with `config.active_record.allow_deprecated_singular_associations_name = false`
+
+    *Adam Hess*
+
 *   Run transactional callbacks on the freshest instance to save a given 
     record within a transaction.
 

activerecord/lib/active_record.rb

@@ -336,6 +336,13 @@ def self.global_executor_concurrency # :nodoc:
   singleton_class.attr_accessor :verify_foreign_keys_for_fixtures
   self.verify_foreign_keys_for_fixtures = false
 
+  ##
+  # :singleton-method:
+  # If true, Rails will continue allowing plural association names in where clauses on singular associations
+  # This behavior will be removed in Rails 7.2.
+  singleton_class.attr_accessor :allow_deprecated_singular_associations_name
+  self.allow_deprecated_singular_associations_name = true
+
   singleton_class.attr_accessor :query_transformers
   self.query_transformers = []
 

activerecord/lib/active_record/table_metadata.rb

@@ -23,7 +23,16 @@ def has_column?(column_name)
     end
 
     def associated_with?(table_name)
-      klass&._reflect_on_association(table_name)
+      if reflection = klass&._reflect_on_association(table_name)
+        reflection
+      elsif ActiveRecord.allow_deprecated_singular_associations_name && reflection = klass&._reflect_on_association(table_name.singularize)
+        ActiveSupport::Deprecation.warn(<<~MSG)
+          In Rails 7.2, referring to singular associations by their plural name will be deprecated.
+          To continue querying `#{table_name.singularize}` use `#{table_name}` instead.
+          You can get the new more performant behavior now by setting config.active_record.allow_deprecated_singular_associations_name = false
+        MSG
+        reflection
+      end
     end
 
     def associated_table(table_name)

guides/source/configuring.md

@@ -67,6 +67,8 @@ Below are the default values associated with each target version. In cases of co
 - [`config.action_controller.allow_deprecated_parameters_hash_equality`](#config-action-controller-allow-deprecated-parameters-hash-equality): `false`
 - [`config.log_file_size`](#config-log-file-size): `100.megabytes`
 - [`config.active_record.sqlite3_adapter_strict_strings_by_default`](#config-active-record-sqlite3-adapter-strict-strings-by-default): `false`
+- [`config.active_record.allow_deprecated_singular_associations_name`](#config-active-record-allow-deprecated-singular-associations-name): `true`
+
 
 #### Default Values for Target Version 7.0
 
@@ -1027,6 +1029,25 @@ Defaults to `4`.
 This number must be considered in accordance with the database pool size configured in `database.yml`. The connection pool
 should be large enough to accommodate both the foreground threads (.e.g web server or job worker threads) and background threads.
 
+#### `config.active_record.allow_deprecated_singular_associations_name`
+
+This maintains the deprecated associations behavior where singular associations can be referred to in where clauses by their plural name. Enable this configuration option to opt into the new behavior.
+
+before,
+
+```ruby
+class Post
+  self.table_name = "blog_posts"
+end
+
+class Comment
+  belongs_to :post
+end
+
+Comment.join(:post).where(posts: { id: 1 }) # deprecated if the table name is not `posts`
+Comment.join(:post).where(post: { id: 1 }) # instead use the relation's name
+```
+
 #### `ActiveRecord::ConnectionAdapters::Mysql2Adapter.emulate_booleans`
 
 Controls whether the Active Record MySQL adapter will consider all `tinyint(1)` columns as booleans. Defaults to `true`.

railties/lib/rails/application/configuration.rb

@@ -264,6 +264,7 @@ def load_defaults(target_version)
 
           if respond_to?(:active_record)
             active_record.run_commit_callbacks_on_first_saved_instances_in_transaction = false
+            active_record.allow_deprecated_singular_associations_name = false
           end
 
           if respond_to?(:action_dispatch)

railties/lib/rails/generators/rails/app/templates/config/initializers/new_framework_defaults_7_1.rb.tt

@@ -44,3 +44,6 @@
 # For example, it is possible to create an index for a non existing column.
 # See https://www.sqlite.org/quirks.html#double_quoted_string_literals_are_accepted for more details.
 # Rails.application.config.active_record.sqlite3_adapter_strict_strings_by_default = true
+
+# Disable deprecated singular associations names
+# Rails.application.config.active_record.allow_deprecated_singular_associations_name = true