Fix breaking with a habtm association with scope given

Fixes #2067

Author: mshibuya

lib/rails_admin/adapters/active_record/association.rb

@@ -92,7 +92,7 @@ def inverse_of
         end
 
         def read_only?
-          (klass.all.instance_eval(&scope).readonly_value if scope.is_a? Proc) ||
+          (klass.all.instance_exec(&scope).readonly_value if scope.is_a?(Proc) && scope.arity == 0) ||
             association.nested? ||
             false
         end

spec/rails_admin/adapters/active_record/association_spec.rb

@@ -20,7 +20,8 @@ class ARPost < Tableless
     end
 
     class ARCategory < Tableless
-      has_and_belongs_to_many :a_r_posts
+      has_and_belongs_to_many :a_r_posts, -> { readonly }
+      has_and_belongs_to_many :scoped_posts, ->(category) { where(id: category.id) }, class_name: 'ARPost'
       belongs_to :librarian, polymorphic: true
     end
 
@@ -173,6 +174,22 @@ class FieldTestWithSymbolForeignKey < FieldTest
       expect(subject.read_only?).to be_falsey
       expect(subject.nested_options).to be_nil
     end
+
+    context 'with a scope given' do
+      subject { @category.associations.detect { |a| a.name == :a_r_posts } }
+
+      it 'does not break' do
+        expect(subject.read_only?).to be_truthy
+      end
+    end
+
+    context 'with a scope that receives an argument given' do
+      subject { @category.associations.detect { |a| a.name == :scoped_posts } }
+
+      it 'ignores the scope' do
+        expect(subject.read_only?).to be_falsey
+      end
+    end
   end
 
   describe 'polymorphic belongs_to association' do