Deprecate mismatched collation comparison for uniquness validator

In MySQL, the default collation is case insensitive. Since the
uniqueness validator enforces case sensitive comparison by default, it
frequently causes mismatched collation issues (performance, weird
behavior, etc) to MySQL users.

https://grosser.it/2009/12/11/validates_uniqness_of-mysql-slow/
https://github.com/rails/rails/issues/1399
https://github.com/rails/rails/pull/13465
https://github.com/gitlabhq/gitlabhq/commit/c1dddf8c7d947691729f6d64a8ea768b5c915855
https://github.com/huginn/huginn/pull/1330#discussion_r55152573

I'd like to deprecate the implicit default enforcing since I frequently
experienced the problems in code reviews.

Note that this change has no effect to sqlite3, postgresql, and
oracle-enhanced adapters which are implemented as case sensitive by
default, only affect to mysql2 adapter (I can take a work if sqlserver
adapter will support Rails 6.0).

6 files changed, 86 insertions, 12 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 42c0e6a190..ec412c2a9d 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,11 @@
+*   Deprecate mismatched collation comparison for uniquness validator.
+
+    Uniqueness validator will no longer enforce case sensitive comparison in Rails 6.1.
+    To continue case sensitive comparison on the case insensitive column,
+    pass `case_sensitive: true` option explicitly to the uniqueness validator.
+
+    *Ryuta Kamizono*
+
 *   Add `reselect` method. This is a short-hand for `unscope(:select).select(fields)`.
 
     Fixes #27340.
diff --git a/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
index f205d77ddb..823efee301 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
@@ -506,7 +506,7 @@ module ActiveRecord
         @connection
       end
 
-      def default_uniqueness_comparison(attribute, value) # :nodoc:
+      def default_uniqueness_comparison(attribute, value, klass) # :nodoc:
         case_sensitive_comparison(attribute, value)
       end
 
diff --git a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
index 37506a97e2..96f902792e 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
@@ -453,6 +453,20 @@ module ActiveRecord
         SQL
       end
 
+      def default_uniqueness_comparison(attribute, value, klass) # :nodoc:
+        column = column_for_attribute(attribute)
+
+        if column.collation && !column.case_sensitive?
+          ActiveSupport::Deprecation.warn(<<~MSG.squish)
+            Uniqueness validator will no longer enforce case sensitive comparison in Rails 6.1.
+            To continue case sensitive comparison on the :#{attribute.name} attribute in #{klass} model,
+            pass `case_sensitive: true` option explicitly to the uniqueness validator.
+          MSG
+        end
+
+        super
+      end
+
       def case_sensitive_comparison(attribute, value) # :nodoc:
         column = column_for_attribute(attribute)
 
diff --git a/activerecord/lib/active_record/validations/uniqueness.rb b/activerecord/lib/active_record/validations/uniqueness.rb
index 88ae62c681..dc1be73a60 100644
--- a/activerecord/lib/active_record/validations/uniqueness.rb
+++ b/activerecord/lib/active_record/validations/uniqueness.rb
@@ -63,7 +63,7 @@ module ActiveRecord
           if bind.nil?
             attr.eq(bind)
           elsif !options.key?(:case_sensitive)
-            klass.connection.default_uniqueness_comparison(attr, bind)
+            klass.connection.default_uniqueness_comparison(attr, bind, klass)
           elsif options[:case_sensitive]
             klass.connection.case_sensitive_comparison(attr, bind)
           else
diff --git a/activerecord/test/cases/validations/uniqueness_validation_test.rb b/activerecord/test/cases/validations/uniqueness_validation_test.rb
index 8f6f47e5fb..301e552642 100644
--- a/activerecord/test/cases/validations/uniqueness_validation_test.rb
+++ b/activerecord/test/cases/validations/uniqueness_validation_test.rb
@@ -314,6 +314,51 @@ class UniquenessValidationTest < ActiveRecord::TestCase
     assert t3.save, "Should save t3 as unique"
   end
 
+  if current_adapter?(:Mysql2Adapter)
+    def test_deprecate_validate_uniqueness_mismatched_collation
+      Topic.validates_uniqueness_of(:author_email_address)
+
+      topic1 = Topic.new(author_email_address: "david@loudthinking.com")
+      topic2 = Topic.new(author_email_address: "David@loudthinking.com")
+
+      assert_equal 1, Topic.where(author_email_address: "david@loudthinking.com").count
+
+      assert_deprecated do
+        assert_not topic1.valid?
+        assert_not topic1.save
+        assert topic2.valid?
+        assert topic2.save
+      end
+
+      assert_equal 2, Topic.where(author_email_address: "david@loudthinking.com").count
+      assert_equal 2, Topic.where(author_email_address: "David@loudthinking.com").count
+    end
+  end
+
+  def test_validate_case_sensitive_uniqueness_by_default
+    Topic.validates_uniqueness_of(:author_email_address)
+
+    topic1 = Topic.new(author_email_address: "david@loudthinking.com")
+    topic2 = Topic.new(author_email_address: "David@loudthinking.com")
+
+    assert_equal 1, Topic.where(author_email_address: "david@loudthinking.com").count
+
+    ActiveSupport::Deprecation.silence do
+      assert_not topic1.valid?
+      assert_not topic1.save
+      assert topic2.valid?
+      assert topic2.save
+    end
+
+    if current_adapter?(:Mysql2Adapter)
+      assert_equal 2, Topic.where(author_email_address: "david@loudthinking.com").count
+      assert_equal 2, Topic.where(author_email_address: "David@loudthinking.com").count
+    else
+      assert_equal 1, Topic.where(author_email_address: "david@loudthinking.com").count
+      assert_equal 1, Topic.where(author_email_address: "David@loudthinking.com").count
+    end
+  end
+
   def test_validate_case_sensitive_uniqueness
     Topic.validates_uniqueness_of(:title, case_sensitive: true, allow_nil: true)
 
diff --git a/activerecord/test/schema/schema.rb b/activerecord/test/schema/schema.rb
index 1dfea0f506..5d46bd47d9 100644
--- a/activerecord/test/schema/schema.rb
+++ b/activerecord/test/schema/schema.rb
@@ -8,6 +8,13 @@ ActiveRecord::Schema.define do
   #                                                                     #
   # ------------------------------------------------------------------- #
 
+  case_sensitive_options =
+    if current_adapter?(:Mysql2Adapter)
+      { collation: "utf8mb4_bin" }
+    else
+      {}
+    end
+
   create_table :accounts, force: true do |t|
     t.references :firm, index: false
     t.string  :firm_name
@@ -266,7 +273,7 @@ ActiveRecord::Schema.define do
   end
 
   create_table :dashboards, force: true, id: false do |t|
-    t.string :dashboard_id
+    t.string :dashboard_id, **case_sensitive_options
     t.string :name
   end
 
@@ -330,7 +337,7 @@ ActiveRecord::Schema.define do
   end
 
   create_table :essays, force: true do |t|
-    t.string :name
+    t.string :name, **case_sensitive_options
     t.string :writer_id
     t.string :writer_type
     t.string :category_id
@@ -338,7 +345,7 @@ ActiveRecord::Schema.define do
   end
 
   create_table :events, force: true do |t|
-    t.string :title, limit: 5
+    t.string :title, limit: 5, **case_sensitive_options
   end
 
   create_table :eyes, force: true do |t|
@@ -380,7 +387,7 @@ ActiveRecord::Schema.define do
   end
 
   create_table :guids, force: true do |t|
-    t.column :key, :string
+    t.column :key, :string, **case_sensitive_options
   end
 
   create_table :guitars, force: true do |t|
@@ -388,8 +395,8 @@ ActiveRecord::Schema.define do
   end
 
   create_table :inept_wizards, force: true do |t|
-    t.column :name, :string, null: false
-    t.column :city, :string, null: false
+    t.column :name, :string, null: false, **case_sensitive_options
+    t.column :city, :string, null: false, **case_sensitive_options
     t.column :type, :string
   end
 
@@ -876,8 +883,8 @@ ActiveRecord::Schema.define do
   end
 
   create_table :topics, force: true do |t|
-    t.string   :title, limit: 250
-    t.string   :author_name
+    t.string   :title, limit: 250, **case_sensitive_options
+    t.string   :author_name, **case_sensitive_options
     t.string   :author_email_address
     if subsecond_precision_supported?
       t.datetime :written_on, precision: 6
@@ -889,10 +896,10 @@ ActiveRecord::Schema.define do
     # use VARCHAR2(4000) instead of CLOB datatype as CLOB data type has many limitations in
     # Oracle SELECT WHERE clause which causes many unit test failures
     if current_adapter?(:OracleAdapter)
-      t.string   :content, limit: 4000
+      t.string   :content, limit: 4000, **case_sensitive_options
       t.string   :important, limit: 4000
     else
-      t.text     :content
+      t.text     :content, **case_sensitive_options
       t.text     :important
     end
     t.boolean  :approved, default: true