1 files changed, 29 insertions, 1 deletions
diff --git a/activerecord/lib/active_record/validations/uniqueness.rb b/activerecord/lib/active_record/validations/uniqueness.rb
index 154cf5f1a4..2677fade18 100644
--- a/activerecord/lib/active_record/validations/uniqueness.rb
+++ b/activerecord/lib/active_record/validations/uniqueness.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module ActiveRecord
   module Validations
     class UniquenessValidator < ActiveModel::EachValidator # :nodoc:
@@ -50,7 +52,33 @@ module ActiveRecord
       end
 
       def build_relation(klass, attribute, value)
-        klass.unscoped.where!({ attribute => value }, options)
+        if reflection = klass._reflect_on_association(attribute)
+          attribute = reflection.foreign_key
+          value = value.attributes[reflection.klass.primary_key] unless value.nil?
+        end
+
+        if value.nil?
+          return klass.unscoped.where!(attribute => value)
+        end
+
+        # the attribute may be an aliased attribute
+        if klass.attribute_alias?(attribute)
+          attribute = klass.attribute_alias(attribute)
+        end
+
+        attribute_name = attribute.to_s
+        value = klass.predicate_builder.build_bind_attribute(attribute_name, value)
+
+        table = klass.arel_table
+        column = klass.columns_hash[attribute_name]
+
+        comparison = if !options[:case_sensitive]
+          # will use SQL LOWER function before comparison, unless it detects a case insensitive collation
+          klass.connection.case_insensitive_comparison(table, attribute, column, value)
+        else
+          klass.connection.case_sensitive_comparison(table, attribute, column, value)
+        end
+        klass.unscoped.where!(comparison)
       end
 
       def scope_relation(record, relation)