Adds support for #except and #only on relations

lib/active_model/relation.rb

@@ -125,6 +125,14 @@ def inspect
       "#<#{self.class.name} [#{entries.join(', ')}]>"
     end
 
+    def except(*skips)
+      relation_with(values.except(*skips))
+    end
+
+    def only(*keeps)
+      relation_with(values.slice(*keeps))
+    end
+
     private
 
     def method_missing(...)
@@ -142,5 +150,21 @@ def respond_to_missing?(...)
     def spawn
       clone
     end
+
+    def values
+      {
+        where: where_clause,
+        offset: offset_value,
+        limit: limit_value
+      }
+    end
+
+    def relation_with(values)
+      spawn.tap do |relation|
+        relation.where_clause = values[:where] || WhereClause.new
+        relation.offset_value = values[:offset]
+        relation.limit_value = values[:limit]
+      end
+    end
   end
 end

spec/active_model/relation_spec.rb

@@ -189,6 +189,52 @@ def doubled_size
     end
   end
 
+  describe '#except' do
+    it 'should return a new relation' do
+      expect(subject.except(:where)).to be_a(described_class)
+    end
+
+    it 'should return a new instance' do
+      expect(subject.except(:where)).not_to eq(subject)
+    end
+
+    it 'should remove the where values' do
+      expect(subject.where(state: 'completed').except(:where)).to match_array(records)
+    end
+
+    it 'should remove the offset value' do
+      expect(subject.offset(1).except(:offset)).to match_array(records)
+    end
+
+    it 'should remove the limit value' do
+      expect(subject.limit(1).except(:limit)).to match_array(records)
+    end
+  end
+
+  describe '#only' do
+    subject { super().where(state: 'completed').offset(1).limit(1) }
+
+    it 'should return a new relation' do
+      expect(subject.only(:where)).to be_a(described_class)
+    end
+
+    it 'should return a new instance' do
+      expect(subject.only(:where)).not_to eq(subject)
+    end
+
+    it 'should keep only the where values' do
+      expect(subject.where(state: 'completed').only(:where)).to match_array(records[2..3])
+    end
+
+    it 'should keep only the offset value' do
+      expect(subject.offset(1).only(:offset)).to match_array(records[1..3])
+    end
+
+    it 'should keep only the limit value' do
+      expect(subject.limit(1).only(:limit)).to match_array(records[0..0])
+    end
+  end
+
   describe 'model class methods' do
     it 'should delegate methods to the model class' do
       expect(subject.model_name).to eq(model_class.model_name)