diff --git a/activerecord/lib/active_record/type/decimal.rb b/activerecord/lib/active_record/type/decimal.rb
index 1c7edc12f5..31c465afdc 100644
--- a/activerecord/lib/active_record/type/decimal.rb
+++ b/activerecord/lib/active_record/type/decimal.rb
@@ -15,7 +15,7 @@ module ActiveRecord
 
       def cast_value(value)
         case value
-        when Numeric, String, Rational
+        when ::Numeric, ::String
           BigDecimal(value, precision.to_i)
         when proc { value.respond_to?(:to_d) }
           value.to_d
diff --git a/activerecord/test/cases/type/decimal_test.rb b/activerecord/test/cases/type/decimal_test.rb
new file mode 100644
index 0000000000..1415afc420
--- /dev/null
+++ b/activerecord/test/cases/type/decimal_test.rb
@@ -0,0 +1,24 @@
+require "cases/helper"
+
+module ActiveRecord
+  module ConnectionAdapters
+    class DecimalTest < ActiveRecord::TestCase
+      def test_type_cast_decimal
+        type = Type::Decimal.new
+        assert_equal BigDecimal.new("0"), type.type_cast_from_user(BigDecimal.new("0"))
+        assert_equal BigDecimal.new("123"), type.type_cast_from_user(123.0)
+        assert_equal BigDecimal.new("1"), type.type_cast_from_user(:"1")
+      end
+
+      def test_type_cast_rational_to_decimal_with_precision
+        type = Type::Decimal.new(precision: 2)
+        assert_equal BigDecimal("0.33"), type.type_cast_from_user(Rational(1, 3))
+      end
+
+      def test_type_cast_rational_to_decimal_without_precision_defaults_to_18_36
+        type = Type::Decimal.new
+        assert_equal BigDecimal("0.333333333333333333E0"), type.type_cast_from_user(Rational(1, 3))
+      end
+    end
+  end
+end
diff --git a/activerecord/test/cases/types_test.rb b/activerecord/test/cases/types_test.rb
index 541c0a4e12..5c54812f30 100644
--- a/activerecord/test/cases/types_test.rb
+++ b/activerecord/test/cases/types_test.rb
@@ -95,23 +95,6 @@ module ActiveRecord
         assert_not type.changed?(nil, nil, nil)
       end
 
-      def test_type_cast_decimal
-        type = Type::Decimal.new
-        assert_equal BigDecimal.new("0"), type.type_cast_from_user(BigDecimal.new("0"))
-        assert_equal BigDecimal.new("123"), type.type_cast_from_user(123.0)
-        assert_equal BigDecimal.new("1"), type.type_cast_from_user(:"1")
-      end
-
-      def test_type_cast_rational_to_decimal_with_precision
-        type = Type::Decimal.new(precision: 2)
-        assert_equal BigDecimal("0.33"), type.type_cast_from_user(Rational(1, 3))
-      end
-
-      def test_type_cast_rational_to_decimal_without_precision_defaults_to_18_36
-        type = Type::Decimal.new
-        assert_equal BigDecimal("0.333333333333333333E0"), type.type_cast_from_user(Rational(1, 3))
-      end
-
       def test_type_cast_binary
         type = Type::Binary.new
         assert_equal nil, type.type_cast_from_user(nil)