Scope in associations should treat nil as `all`

Defined scope treats nil as `all`, but scope in associations isn't so.
If the result of the scope is nil, most features on associations will be
broken. It should treat nil as `all` like defined scope.

Fixes #20823.

6 files changed, 11 insertions, 9 deletions

diff --git a/activerecord/lib/active_record/associations/association.rb b/activerecord/lib/active_record/associations/association.rb
index 268b022ab8..ca1f9f1650 100644
--- a/activerecord/lib/active_record/associations/association.rb
+++ b/activerecord/lib/active_record/associations/association.rb
@@ -130,8 +130,8 @@ module ActiveRecord
       def extensions
         extensions = klass.default_extensions | reflection.extensions
 
-        if scope = reflection.scope
-          extensions |= klass.unscoped.instance_exec(owner, &scope).extensions
+        if reflection.scope
+          extensions |= reflection.scope_for(klass.unscoped, owner).extensions
         end
 
         extensions
diff --git a/activerecord/lib/active_record/associations/association_scope.rb b/activerecord/lib/active_record/associations/association_scope.rb
index 3d79e540b8..e790bd3db9 100644
--- a/activerecord/lib/active_record/associations/association_scope.rb
+++ b/activerecord/lib/active_record/associations/association_scope.rb
@@ -174,7 +174,8 @@ module ActiveRecord
         end
 
         def eval_scope(reflection, table, scope, owner)
-          reflection.build_scope(table).instance_exec(owner, &scope)
+          relation = reflection.build_scope(table)
+          relation.instance_exec(owner, &scope) || relation
         end
     end
   end
diff --git a/activerecord/lib/active_record/associations/join_dependency.rb b/activerecord/lib/active_record/associations/join_dependency.rb
index ac37c3c015..77b3d11b47 100644
--- a/activerecord/lib/active_record/associations/join_dependency.rb
+++ b/activerecord/lib/active_record/associations/join_dependency.rb
@@ -256,7 +256,8 @@ module ActiveRecord
             else
               model = construct_model(ar_parent, node, row, model_cache, id, aliases)
 
-              if node.reflection.scope_for(node.base_klass).readonly_value
+              if node.reflection.scope &&
+                  node.reflection.scope_for(node.base_klass.unscoped).readonly_value
                 model.readonly!
               end
 
diff --git a/activerecord/lib/active_record/associations/preloader/association.rb b/activerecord/lib/active_record/associations/preloader/association.rb
index 4915a37f06..3fa97af759 100644
--- a/activerecord/lib/active_record/associations/preloader/association.rb
+++ b/activerecord/lib/active_record/associations/preloader/association.rb
@@ -113,7 +113,7 @@ module ActiveRecord
           end
 
           def reflection_scope
-            @reflection_scope ||= reflection.scope_for(klass)
+            @reflection_scope ||= reflection.scope ? reflection.scope_for(klass.unscoped) : klass.unscoped
           end
 
           def build_scope
diff --git a/activerecord/lib/active_record/reflection.rb b/activerecord/lib/active_record/reflection.rb
index 00ff20f4d7..49ddcaecdf 100644
--- a/activerecord/lib/active_record/reflection.rb
+++ b/activerecord/lib/active_record/reflection.rb
@@ -214,7 +214,7 @@ module ActiveRecord
 
       def join_scopes(table, predicate_builder) # :nodoc:
         if scope
-          [build_scope(table, predicate_builder).instance_exec(&scope)]
+          [scope_for(build_scope(table, predicate_builder))]
         else
           []
         end
@@ -391,8 +391,8 @@ module ActiveRecord
           active_record == other_aggregation.active_record
       end
 
-      def scope_for(klass)
-        scope ? klass.unscoped.instance_exec(nil, &scope) : klass.unscoped
+      def scope_for(relation, owner = nil)
+        relation.instance_exec(owner, &scope) || relation
       end
 
       private
diff --git a/activerecord/test/models/author.rb b/activerecord/test/models/author.rb
index 025d013fb4..0a52cc5390 100644
--- a/activerecord/test/models/author.rb
+++ b/activerecord/test/models/author.rb
@@ -78,7 +78,7 @@ class Author < ActiveRecord::Base
            after_add: [:log_after_adding,  Proc.new { |o, r| o.post_log << "after_adding_proc#{r.id || '<new>'}" }]
   has_many :unchangeable_posts, class_name: "Post", before_add: :raise_exception, after_add: :log_after_adding
 
-  has_many :categorizations
+  has_many :categorizations, -> {}
   has_many :categories, through: :categorizations
   has_many :named_categories, through: :categorizations