db={"users":[{name:"John",user_type:"client",society:1},{name:"Charles",user_type:"client",society:1},{name:"Jessy",user_type:"provider",society:1},{name:"Tim",user_type:"provider",society:2}],"clients":[{_id:1,name:"Client1"}],"providers":[{_id:1,name:"Provider1"},{_id:2,name:"Provider2"}]}

db.users.aggregate([{"$lookup":{"from":"clients","localField":"society","foreignField":"_id","as":"client"}},{"$lookup":{"from":"providers","localField":"society","foreignField":"_id","as":"provider"}},{$project:{name:1,society:1,user_type:1,complete_society:{$arrayElemAt:[{$cond:[{$eq:["$user_type","client"]},"$client","$provider"]},0]}}}])