Add `null: false` constraint by default for `belongs_to` associations

- Also deprecate passing {required} to the model generator.
- Also made sure the global config `belongs_to_required_by_default` is
  applied correctly to the model generator for `null: false` option.

1 files changed, 8 insertions, 2 deletions

diff --git a/railties/lib/rails/generators/generated_attribute.rb b/railties/lib/rails/generators/generated_attribute.rb
index 99c1bc4269..1a80e71eae 100644
--- a/railties/lib/rails/generators/generated_attribute.rb
+++ b/railties/lib/rails/generators/generated_attribute.rb
@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "active_support/time"
+require "active_support/deprecation"
 
 module Rails
   module Generators
@@ -51,6 +52,12 @@ module Rails
               type = $1
               provided_options = $2.split(/[,.-]/)
               options = Hash[provided_options.map { |opt| [opt.to_sym, true] }]
+
+              if options[:required]
+                ActiveSupport::Deprecation.warn("Passing {required} option has no effect on the model generator. It will be removed in Rails 6.1.\n")
+                options.delete(:required)
+              end
+
               return type, options
             else
               return type, {}
@@ -137,7 +144,7 @@ module Rails
       end
 
       def required?
-        attr_options[:required]
+        reference? && Rails.application.config.active_record.belongs_to_required_by_default
       end
 
       def has_index?
@@ -183,7 +190,6 @@ module Rails
       def options_for_migration
         @attr_options.dup.tap do |options|
           if required?
-            options.delete(:required)
             options[:null] = false
           end