[Fix #1319] Fix false positive for RedundantPresenceValidationOnBelongsTo

If presence is the only validation option and other non-validation options
are present, removing it will cause rails to error.

Author: Earlopain

changelog/fix_false_positive_for_rails_redundant_presence_validation_on_belongs_to.md

@@ -0,0 +1 @@
+* [#1319](https://github.com/rubocop/rubocop-rails/issues/1319): Fix a false positive for `Rails/RedundantPresenceValidationOnBelongsTo` when removing `presence` would leave other non-validation options like `allow_blank` without validations. ([@earlopain][])

lib/rubocop/cop/rails/redundant_presence_validation_on_belongs_to.rb

@@ -39,6 +39,9 @@ class RedundantPresenceValidationOnBelongsTo < Base
         MSG = 'Remove explicit presence validation for %<association>s.'
         RESTRICT_ON_SEND = %i[validates].freeze
 
+        # From https://github.com/rails/rails/blob/7a0bf93b9dd291c7f61121a41b3a813ac8857e6a/activemodel/lib/active_model/validations/validates.rb#L157-L159
+        NON_VALIDATION_OPTIONS = %i[if unless on allow_blank allow_nil strict].freeze
+
         minimum_target_rails_version 5.0
 
         # @!method presence_validation?(node)
@@ -170,6 +173,12 @@ class RedundantPresenceValidationOnBelongsTo < Base
 
         def on_send(node)
           presence_validation?(node) do |all_keys, options, presence|
+            # If presence is the only validation option and other non-validation options
+            # are present, removing it will cause rails to error.
+            used_option_keys = options.keys.select(&:sym_type?).map(&:value)
+            remaining_validations = used_option_keys - NON_VALIDATION_OPTIONS - [:presence]
+            return if remaining_validations.none? && options.keys.length > 1
+
             keys = non_optional_belongs_to(node.parent, all_keys)
             return if keys.none?
 

spec/rubocop/cop/rails/redundant_presence_validation_on_belongs_to_spec.rb

@@ -278,6 +278,13 @@ class Profile
       RUBY
     end
 
+    it 'does not register an offense with `if` and other validation option' do
+      expect_no_offenses(<<~RUBY)
+        belongs_to :user
+        validates :user, presence: true, if: -> { condition }, numericality: true
+      RUBY
+    end
+
     it 'does not register an offense with `unless` option' do
       expect_no_offenses(<<~RUBY)
         belongs_to :user
@@ -298,6 +305,13 @@ class Profile
         validates :user, presence: true, strict: MissingUserError
       RUBY
     end
+
+    it 'does not register an offense when other options are present but none are validations' do
+      expect_no_offenses(<<~RUBY)
+        belongs_to :user
+        validates :user, presence: true, allow_blank: false
+      RUBY
+    end
   end
 
   context 'Rails < 5.0', :rails42 do