Move all Arel constructions from uniqueness validator into connection adapter

2 files changed, 5 insertions, 4 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: