Resolve proc/symbol inclusion :in keys to values in OPTIONS.

lib/rest_framework/controller_mixins/models.rb

@@ -266,6 +266,13 @@ def get_fields_metadata
           # Update `required` if we find a presence validator.
           metadata[:required] = true if kind == :presence
 
+          # Resolve procs (and lambdas), and symbols for certain arguments.
+          if options[:in].is_a?(Proc)
+            options = options.merge(in: options[:in].call)
+          elsif options[:in].is_a?(Symbol)
+            options = options.merge(in: model.send(options[:in]))
+          end
+
           metadata[:validators] ||= {}
           metadata[:validators][kind] ||= []
           metadata[:validators][kind] << options

test/app/controllers/test_api_controller.rb

@@ -1,6 +1,6 @@
 class TestApiController < ApplicationController
   DESCRIPTION = <<~TEXT.lines.map(&:strip).join(" ")
-    The test API contains a lot of really weird controllers for unit testing specific features.
+    The test API contains a lot of really weird controllers for testing specific features.
   TEXT
 
   include RESTFramework::BaseControllerMixin

test/app/models/user.rb

@@ -45,7 +45,16 @@ class User < ApplicationRecord
 
   validates_numericality_of :balance, greater_than: 0, allow_nil: true
   validates_inclusion_of :state, in: states.keys
-  validates_inclusion_of :status, in: STATUS_OPTS.keys
+  validates_inclusion_of :status, in: :status_keys
+
+  # Adding this class method to test serializing inclusion symbol in `OPTIONS` endpoint.
+  def self.status_keys
+    return STATUS_OPTS.keys
+  end
+
+  def status_keys
+    return self.class.status_keys
+  end
 
   # An example of a "calculated" property method.
   def calculated_property