db={"events":[{"_id":ObjectId("5f6c5b0594911c0b7231643d"),"title":"Test Event","tags":["5fd8cef2368c625a9d7516cb"]},{"_id":ObjectId("5fb3896afc13ae2ed1000000"),"title":"Test Event 2","tags":["5fd8cef2368c625a9d7516cb","5fd8cfb04a4a6063ab4ca4bf","5fd8cfb04a4a6063ab4ca4c0"]}],"tags":[{"_id":ObjectId("5fd8cef2368c625a9d7516cb"),"name":"Foo","createdAt":ISODate("2020-12-15T14:57:54.096Z")},{"_id":ObjectId("5fd8cfb04a4a6063ab4ca4bf"),"name":"Bar","createdBy":"embo@team-con.de","createdAt":ISODate("2020-12-16T14:57:54.096Z")},{"_id":ObjectId("5fd8cfb04a4a6063ab4ca4c0"),"name":"Foobar","createdAt":ISODate("2020-12-17T14:57:54.096Z")}]}

db.events.aggregate([{$match:{tags:{$exists:true}}},{$addFields:{tags:{$map:{input:"$tags",in:{$toObjectId:"$$this"}}}}},{$lookup:{from:"tags",localField:"tags",foreignField:"_id",as:"tags"}}])