Database
[{config:{_id:ObjectId("63c6a4b858612f44f37d4771"),type:"TypeA",priority:1}},{config:{_id:ObjectId("627279d3ba7aef5d6418c867"),type:"TypeB",priority:3}},{config:{_id:ObjectId("628b4d3ff5b1c1736c0b654a"),type:"TypeC",priority:4}}]
Query
db.collection.aggregate([{$group:{_id:null,data:{$push:{k:"$config.type",v:{_id:"$config._id",priority:"$config.priority"}}}}},{$replaceRoot:{newRoot:{$arrayToObject:"$data"}}}])