db={"users":[{"_id":{"$oid":"630273bbe0f3f82d85b149bd"},"password":"$2b$12$2f04ijl0fDVHUgBF3rZi.eKfNYkzHZShHNPKfiDEu2NXEXhb.2O","name":"name1","tel":"+33612345678"}],"leads":[{"_id":{"$oid":"120273bbe0f3f82d85b149ba"},"name":"name2","tel":"+33712345679"},{"_id":{"$oid":"987273bbe0f3f82d81b149ba"},"name":"name1","tel":"+33612345678"}],"bookings":[{"_id":{"$oid":"1360a36c6303d0987ce6fac4"},"date":{"$date":"2022-10-01T14:36:12Z"},"user":{"$oid":"987273bbe0f3f82d81b149ba"},"ref":"Lead"},{"_id":{"$oid":"1360a86c6303d0987ce6fac4"},"date":{"$date":"2022-10-01T14:36:12Z"},"user":{"$oid":"120273bbe0f3f82d85b149ba"},"ref":"Lead"},{"_id":{"$oid":"6310a86c6303d0987fe6fac4"},"date":{"$date":"2022-09-01T14:36:12Z"},"user":{"$oid":"630273bbe0f3f82d85b149bd"},"ref":"User"}]}

db.bookings.aggregate([{$group:{_id:{user:"$user"}}},{$lookup:{from:"users",localField:"_id.user",foreignField:"_id",as:"leadUser",pipeline:[{$project:{name:1,tel:1}}]}},{$lookup:{from:"leads",localField:"_id.user",foreignField:"_id",as:"lead",pipeline:[{$project:{name:1,tel:1}}]}},{$project:{lead:{$first:{$concatArrays:["$lead","$leadUser"]}}}},{$group:{_id:"$lead.tel",lead:{$first:"$lead"},user:{$first:"$_id.user"}}},{$project:{_id:{user:"$user"},lead:1,}}])