Use non-exist enum string to get unrelated record in My SQL

This behaviour is in
rails/activerecord/lib/active_record/enum.rb #serialize(value) line no 143
if value is not present in mapping we are sending the value back ,
which in mysql returns unrelated record.

I have changed to return nil is value is not present in mapping

Implemented code review changes

Improved test case coverage

[ci skip] - cosmetic changes for better readibility of change log

Signed-off-by: ak <atulkanswal@gmail.com>

activerecord/CHANGELOG.md

@@ -1,3 +1,16 @@
+* Raise error when non-existent enum used in query
+  
+  This change will raise an error when a non-existent enum is passed to a query. Previously
+  with MySQL this would return an unrelated record. Fixes #38687.
+    ```ruby
+    class User < ActiveRecord::Base
+      enum status: { active: 0, non_active: 1 }
+    end
+    User.where(status: :non_existing_status)
+    => ArgumentError ('non_existing_status' is not a valid status)
+    ```
+  *Atul Kanswal *
+
 *   Dump the schema or structure of a database when calling db:migrate:name
 
     In previous versions of Rails, `rails db:migrate` would dump the schema of the database. In Rails 6, that holds true (`rails db:migrate` dumps all databases' schemas), but `rails db:migrate:name` does not share that behavior.

activerecord/lib/active_record/enum.rb

@@ -140,6 +140,7 @@ def deserialize(value)
       end
 
       def serialize(value)
+        assert_valid_value(value)
         mapping.fetch(value, value)
       end
 

activerecord/test/cases/enum_test.rb

@@ -59,6 +59,7 @@ class EnumTest < ActiveRecord::TestCase
     assert_not_equal @book, Book.where(status: [written]).first
     assert_not_equal @book, Book.where("status <> ?", published).first
     assert_equal @book, Book.where("status <> ?", written).first
+    assert_empty Book.where(status: nil)
   end
 
   test "find via where with symbols" do
@@ -69,6 +70,8 @@ class EnumTest < ActiveRecord::TestCase
     assert_not_equal @book, Book.where.not(status: :published).first
     assert_equal @book, Book.where.not(status: :written).first
     assert_equal books(:ddd), Book.where(read_status: :forgotten).first
+    exception = assert_raises(ArgumentError) { Book.where(status: :not_defined).first }
+    assert_match(/'not_defined' is not a valid status/, exception.message)
   end
 
   test "find via where with strings" do
@@ -79,6 +82,8 @@ class EnumTest < ActiveRecord::TestCase
     assert_not_equal @book, Book.where.not(status: "published").first
     assert_equal @book, Book.where.not(status: "written").first
     assert_equal books(:ddd), Book.where(read_status: "forgotten").first
+    exception = assert_raises(ArgumentError) { Book.where(status: "not_defined").first }
+    assert_match(/'not_defined' is not a valid status/, exception.message)
   end
 
   test "build from scope" do