Implement Association#foreign_key_nullable?

lib/rails_admin/adapters/active_record/association.rb

@@ -37,6 +37,11 @@ def foreign_key
           association.foreign_key.to_sym
         end
 
+        def foreign_key_nullable?
+          return if foreign_key.nil? or type != :has_many
+          klass.columns_hash[association.foreign_key].null
+        end
+
         def foreign_type
           options[:foreign_type].try(:to_sym) || :"#{name}_type" if options[:polymorphic]
         end

lib/rails_admin/adapters/mongoid/association.rb

@@ -48,6 +48,11 @@ def foreign_key
           association.foreign_key.to_sym rescue nil
         end
 
+        def foreign_key_nullable?
+          return if foreign_key.nil?
+          true
+        end
+
         def foreign_type
           return unless polymorphic? && [:referenced_in, :belongs_to].include?(macro)
           association.inverse_type.try(:to_sym) || :"#{name}_type"

spec/rails_admin/adapters/active_record/association_spec.rb

@@ -103,6 +103,7 @@ class ARComment < ActiveRecord::Base
         expect(association.type).to eq :has_many
         expect(association.klass).to eq Division
         expect(association.read_only?).to be_falsey
+        expect(association.foreign_key_nullable?).to be_truthy
       end
     end
 
@@ -113,6 +114,7 @@ class ARComment < ActiveRecord::Base
         expect(association.type).to eq :has_many
         expect(association.klass).to eq Team
         expect(association.read_only?).to be_truthy
+        expect(association.foreign_key_nullable?).to be_truthy
       end
     end
 
@@ -127,6 +129,18 @@ class ARComment < ActiveRecord::Base
     end
   end
 
+  describe 'has_many association with not nullable foreign key' do
+    let(:field_test) { RailsAdmin::AbstractModel.new(FieldTest) }
+
+    context 'for direct has many' do
+      let(:association) { field_test.associations.detect { |a| a.name == :nested_field_tests } }
+
+      it 'returns correct values' do
+        expect(association.foreign_key_nullable?).to be_falsey
+      end
+    end
+  end
+
   describe 'has_and_belongs_to_many association' do
     subject { @post.associations.detect { |a| a.name == :a_r_categories } }
 
@@ -135,6 +149,7 @@ class ARComment < ActiveRecord::Base
       expect(subject.klass).to eq ARCategory
       expect(subject.primary_key).to eq :id
       expect(subject.foreign_type).to be_nil
+      expect(subject.foreign_key_nullable?).to be_nil
       expect(subject.as).to be_nil
       expect(subject.polymorphic?).to be_falsey
       expect(subject.inverse_of).to be_nil

spec/rails_admin/adapters/mongoid/association_spec.rb

@@ -87,6 +87,7 @@ class MongoNote
       expect(subject.klass).to eq MongoBlog
       expect(subject.primary_key).to eq :_id
       expect(subject.foreign_key).to eq :mongo_blog_id
+      expect(subject.foreign_key_nullable?).to be_truthy
       expect(subject.foreign_type).to be_nil
       expect(subject.foreign_inverse_of).to be_nil
       expect(subject.as).to be_nil
@@ -111,6 +112,7 @@ class MongoNote
       expect(subject.klass).to eq MongoPost
       expect(subject.primary_key).to eq :_id
       expect(subject.foreign_key).to eq :mongo_blog_id
+      expect(subject.foreign_key_nullable?).to be_truthy
       expect(subject.foreign_type).to be_nil
       expect(subject.foreign_inverse_of).to be_nil
       expect(subject.as).to be_nil
@@ -130,6 +132,7 @@ class MongoNote
       expect(subject.klass).to eq MongoCategory
       expect(subject.primary_key).to eq :_id
       expect(subject.foreign_key).to eq :mongo_category_ids
+      expect(subject.foreign_key_nullable?).to be_truthy
       expect(subject.foreign_type).to be_nil
       expect(subject.foreign_inverse_of).to be_nil
       expect(subject.as).to be_nil
@@ -150,6 +153,7 @@ class MongoNote
       expect(subject.klass).to eq [MongoBlog, MongoPost]
       expect(subject.primary_key).to eq :_id
       expect(subject.foreign_key).to eq :commentable_id
+      expect(subject.foreign_key_nullable?).to be_truthy
       expect(subject.foreign_type).to eq :commentable_type
       expect(subject.foreign_inverse_of).to be_nil
       expect(subject.as).to be_nil
@@ -170,6 +174,7 @@ class MongoNote
       expect(subject.klass).to eq MongoComment
       expect(subject.primary_key).to eq :_id
       expect(subject.foreign_key).to eq :commentable_id
+      expect(subject.foreign_key_nullable?).to be_truthy
       expect(subject.foreign_type).to be_nil
       expect(subject.foreign_inverse_of).to be_nil
       expect(subject.as).to eq :commentable
@@ -194,6 +199,7 @@ class MongoNote
       expect(subject.klass).to eq MongoNote
       expect(subject.primary_key).to eq :_id
       expect(subject.foreign_key).to be_nil
+      expect(subject.foreign_key_nullable?).to be_nil
       expect(subject.foreign_type).to be_nil
       expect(subject.foreign_inverse_of).to be_nil
       expect(subject.as).to be_nil
@@ -213,6 +219,7 @@ class MongoNote
       expect(subject.klass).to eq MongoNote
       expect(subject.primary_key).to eq :_id
       expect(subject.foreign_key).to be_nil
+      expect(subject.foreign_key_nullable?).to be_nil
       expect(subject.foreign_type).to be_nil
       expect(subject.as).to be_nil
       expect(subject.polymorphic?).to be_falsey