db={setting:[{"_id":ObjectId("5ebb74ef92033b3dc79aca13"),"content":"Here is new content","data":[{"_id":ObjectId("5e7cd2019b1c522b15cca6fe"),"url":"xyz.jpg","width":1000,"height":1000},{"_id":ObjectId("5e7cd2019b1c522b15cca6fd"),"url":"verr.jpg","width":601,"height":1000}],"type":"post","timestamp":1589343471770}],site:[{"_id":ObjectId("5ebcf2b692033b3dc79acd1a"),"text":"Hi Dave","post_id":ObjectId("5ebb74ef92033b3dc79aca13"),"doc_id":ObjectId("5e7cd2019b1c522b15cca6fe"),"sender_id":ObjectId("5e4e74eb380054797d9db623"),"date":1589441206774},{"_id":ObjectId("5ebcf2b692033b3dc79acd4a"),"text":"Hi Dave","post_id":ObjectId("5ebb74ef92033b3dc79aca13"),"doc_id":ObjectId("5e7cd2019b1c522b15cca6fe"),"sender_id":ObjectId("5e4e74eb380054797d9db623"),"date":1589441206776}]}

db.setting.aggregate([{$lookup:{from:"site",let:{p_id:"$_id",d_id:"$data[0]_id",},pipeline:[{$match:{$expr:{$and:[{$eq:["$_id","$$p_id"]},{$eq:["$data_id","$$d_id"]}]}}}],as:"subComment"}},{$project:{_id:1,sub_comments_count:{$size:"$subComment"}}}])