MONGOID-5360 Warn when defining field types of BSON::Int64/Decimal128 and other unsupported types (#5291)

* MONGOID-5360 warn on BSON::* types and add docs

* MONGOID-5360 only warn on Int32/64 and Decimal128

* MONGOID-5360 reword the warning

* MONGOID-5360 reword warning

* Update lib/mongoid/fields.rb

* Update lib/mongoid/fields.rb

* MONGOID-5360 update message and freeze array

Author: neilshweky

docs/reference/fields.txt

@@ -89,6 +89,13 @@ Mongoid also recognizes the string ``"Boolean"`` as an alias for the
 
 To define custom field types, refer to :ref:`Custom Field Types <custom-field-types>` below.
 
+.. note::
+
+  Using the ``BSON::Int64`` and ``BSON::Int32`` types as field types is unsupported.
+  Saving these types to the database will work as expected, however, querying them
+  will return the native Ruby ``Integer`` type. Querying fields of type
+  ``BSON::Decimal128`` will return values of type ``BSON::Decimal128`` in
+  BSON <=4 and values of type ``BigDecimal`` in BSON 5+.
 
 .. _omitting-field-type-definition:
 

lib/mongoid/fields.rb

@@ -42,6 +42,11 @@ module Fields
     # @api private
     IDS = [ :_id, '_id', ].freeze
 
+    # BSON classes that are not supported as field types
+    #
+    # @api private
+    INVALID_BSON_CLASSES = [ BSON::Decimal128, BSON::Int32, BSON::Int64 ].freeze
+
     module ClassMethods
       # Returns the list of id fields for this model class, as both strings
       # and symbols.
@@ -731,8 +736,18 @@ def field_for(name, options)
         opts = options.merge(klass: self)
         type_mapping = TYPE_MAPPINGS[options[:type]]
         opts[:type] = type_mapping || unmapped_type(options)
-        unless opts[:type].is_a?(Class)
+        if !opts[:type].is_a?(Class)
           raise Errors::InvalidFieldType.new(self, name, options[:type])
+        else
+          if INVALID_BSON_CLASSES.include?(opts[:type])
+            warn_message = "Using #{opts[:type]} as the field type is not supported. "
+            if opts[:type] == BSON::Decimal128
+              warn_message += "In BSON <= 4, the BSON::Decimal128 type will work as expected for both storing and querying, but will return a BigDecimal on query in BSON 5+."
+            else
+              warn_message += "Saving values of this type to the database will work as expected, however, querying them will return a value of the native Ruby Integer type."
+            end
+            Mongoid.logger.warn(warn_message)
+          end
         end
         return Fields::Localized.new(name, opts) if options[:localize]
         return Fields::ForeignKey.new(name, opts) if options[:identity]