#foreign_key_nullable? breaks with has_many through

Fixes railsadminteam#2474, closes railsadminteam#2478

lib/rails_admin/adapters/active_record/association.rb

@@ -38,8 +38,8 @@ def foreign_key
         end
 
         def foreign_key_nullable?
-          return if foreign_key.nil? || type != :has_many
-          klass.columns_hash[foreign_key.to_s].null
+          return true if foreign_key.nil? || type != :has_many
+          (column = klass.columns_hash[foreign_key.to_s]).nil? || column.null
         end
 
         def foreign_type

spec/rails_admin/adapters/active_record/association_spec.rb

@@ -161,7 +161,7 @@ class FieldTestWithSymbolForeignKey < FieldTest
       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_falsey
+      expect(subject.foreign_key_nullable?).to be_truthy
       expect(subject.as).to be_nil
       expect(subject.polymorphic?).to be_falsey
       expect(subject.inverse_of).to be_nil

spec/rails_admin/config/fields/association_spec.rb

@@ -14,4 +14,40 @@
       end
     end
   end
+
+  describe '#removable?', active_record: true do
+    context 'with non-nullable foreign key' do
+      let(:field) { RailsAdmin.config('FieldTest').fields.detect { |f| f.name == :nested_field_tests } }
+      it 'is false' do
+        expect(field.removable?).to be false
+      end
+    end
+
+    context 'with nullable foreign key' do
+      let(:field) { RailsAdmin.config('Team').fields.detect { |f| f.name == :players } }
+      it 'is true' do
+        expect(field.removable?).to be true
+      end
+    end
+
+    context 'with polymorphic has_many' do
+      let(:field) { RailsAdmin.config('Player').fields.detect { |f| f.name == :comments } }
+      it 'does not break' do
+        expect(field.removable?).to be true
+      end
+    end
+
+    context 'with has_many through' do
+      before do
+        class TeamWithHasManyThrough < Team
+          has_many :drafts
+          has_many :draft_players, through: :drafts, source: :player
+        end
+      end
+      let(:field) { RailsAdmin.config('TeamWithHasManyThrough').fields.detect { |f| f.name == :draft_players } }
+      it 'does not break' do
+        expect(field.removable?).to be true
+      end
+    end
+  end
 end