Database
db={"Item":[{"_id":ObjectId("5e6f98e41b70ea2c14c208b9"),"name":"test"}],"Data":[{"_id":ObjectId("5e6f99431b70ea2c14c208ba"),"item":ObjectId("5e6f98e41b70ea2c14c208b9"),"grouping_1":[{"name":"A"},{"name":"B"}],"grouping_2":[{"name":"C"},{"name":"D"}]}]}
Query
db.Item.aggregate([{$match:{_id:ObjectId("5e6f98e41b70ea2c14c208b9")}},{$lookup:{from:"Data",localField:"_id",foreignField:"item",as:"data"}},{$project:{names:{$map:{input:{$concatArrays:[{$arrayElemAt:["$data.grouping_1",0]},{$arrayElemAt:["$data.grouping_2",0]}]},in:"$$this.name"}}}}])