db={chats:[{title:"example 1",country:"UK",chat:[{details:"chat1 infos",channel:["5ea3257d4b861a5aa0d03111","5ea3257d4b861a5aa0d03222"]},{details:"chat2 infos",channel:["5ea3257d4b861a5aa0d03333"]}]},{title:"example 2",country:"USA",chat:[{details:"chat3 infos",channel:["5ea3257d4b861a5aa0d03222"]},{details:"chat4 infos",channel:["5ea3257d4b861a5aa0d03333"]}]}],channels:[{_id:"5ea3257d4b861a5aa0d03111",title:"Channel1",details:"Channel1 Details"},{_id:"5ea3257d4b861a5aa0d03222",title:"Channel2",details:"Channel2 Details"},{_id:"5ea3257d4b861a5aa0d03333",title:"Channel3",details:"Channel13 Details"}]}

db.chats.aggregate([{$unwind:"$chat"},{$lookup:{from:"channels",let:{channelIds:"$chat.channel"},pipeline:[{$match:{$expr:{$in:["$_id","$$channelIds"]}}}],as:"chat.channel"}},{$group:{_id:"$_id",title:{$first:"$title"},country:{$first:"$country"},chat:{$push:"$chat"}}}])