Move all Arel constructions from uniqueness validator into connection adapter

3 files changed, 6 insertions, 7 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
index c2087b8216..202187a047 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
@@ -522,7 +522,7 @@ module ActiveRecord
       end
 
       def default_uniqueness_comparison(attribute, value, klass) # :nodoc:
-        case_sensitive_comparison(attribute, value)
+        attribute.eq(value)
       end
 
       def case_sensitive_comparison(attribute, value) # :nodoc:
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 d99c9b9d02..a518b897a0 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
@@ -461,15 +461,16 @@ module ActiveRecord
       def default_uniqueness_comparison(attribute, value, klass) # :nodoc:
         column = column_for_attribute(attribute)
 
-        if column.collation && !column.case_sensitive?
+        if column.collation && !column.case_sensitive? && !value.nil?
           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
+          attribute.eq(Arel::Nodes::Bin.new(value))
+        else
+          super
         end
-
-        super
       end
 
       def case_sensitive_comparison(attribute, value) # :nodoc:
diff --git a/activerecord/lib/active_record/validations/uniqueness.rb b/activerecord/lib/active_record/validations/uniqueness.rb
index dc1be73a60..3172aa2078 100644
--- a/activerecord/lib/active_record/validations/uniqueness.rb
+++ b/activerecord/lib/active_record/validations/uniqueness.rb
@@ -60,9 +60,7 @@ module ActiveRecord
         comparison = relation.bind_attribute(attribute, value) do |attr, bind|
           return relation.none! if bind.unboundable?
 
-          if bind.nil?
-            attr.eq(bind)
-          elsif !options.key?(:case_sensitive)
+          if !options.key?(:case_sensitive) || bind.nil?
             klass.connection.default_uniqueness_comparison(attr, bind, klass)
           elsif options[:case_sensitive]
             klass.connection.case_sensitive_comparison(attr, bind)