Ensure casting by decimal attribute when querying

Related 34cc301f03aea2e579d6687a9ea9782afc1089a0.

`QueryAttribute#value_for_database` calls only `type.serialize`, and
`Decimal#serialize` is a no-op unlike other attribute types.

Whether or not `serialize` will invoke `cast` is undefined in our test
cases, but it actually does not work properly unless it does so for now.

2 files changed, 12 insertions, 2 deletions

diff --git a/activemodel/lib/active_model/type/decimal.rb b/activemodel/lib/active_model/type/decimal.rb
index e8ee18c00e..b37dad1c41 100644
--- a/activemodel/lib/active_model/type/decimal.rb
+++ b/activemodel/lib/active_model/type/decimal.rb
@@ -12,6 +12,10 @@ module ActiveModel
         :decimal
       end
 
+      def serialize(value)
+        cast(value)
+      end
+
       def type_cast_for_schema(value)
         value.to_s.inspect
       end
diff --git a/activerecord/test/cases/numeric_data_test.rb b/activerecord/test/cases/numeric_data_test.rb
index 304714979c..079e664ee4 100644
--- a/activerecord/test/cases/numeric_data_test.rb
+++ b/activerecord/test/cases/numeric_data_test.rb
@@ -24,7 +24,10 @@ class NumericDataTest < ActiveRecord::TestCase
     )
     assert m.save
 
-    m1 = NumericData.find(m.id)
+    m1 = NumericData.find_by(
+      bank_balance: 1586.43,
+      big_bank_balance: BigDecimal("1000234000567.95")
+    )
 
     assert_kind_of Integer, m1.world_population
     assert_equal 2**62, m1.world_population
@@ -48,7 +51,10 @@ class NumericDataTest < ActiveRecord::TestCase
     )
     assert m.save
 
-    m1 = NumericData.find(m.id)
+    m1 = NumericData.find_by(
+      bank_balance: 1586.43122334,
+      big_bank_balance: BigDecimal("234000567.952344")
+    )
 
     assert_kind_of Integer, m1.world_population
     assert_equal 2**62, m1.world_population