[{frameProfileId:"366",sections:[{title:"This is a bogus title",count:1,metadata:{subType:"Quaternion",edgeCount:1}},{title:"Plan b",count:6,metadata:{subType:"Quaternion",edgeCount:1}}]},{frameProfileId:"388",sections:[{title:"Another title",count:14,metadata:{subType:"Quaternion",edgeCount:1}}]}]

db.collection.aggregate([{$project:{sections:{$map:{input:{$filter:{input:"$sections",as:"frameSection",cond:{$eq:["$$frameSection.metadata.subType","Quaternion"]}}},in:{$mergeObjects:["$$this",{frame_id:"$frameProfileId"}]}}}}}])