db={user:[{"_id":ObjectId("8B0F857017EA90AA0042C33B"),"address":{"current_address":{"city":"Thane"}},"roles":["patient","doctor"]},{"_id":ObjectId("5B103AF02C975AC3207AF88D"),"address":{"current_address":{"city":"Pune"}},"roles":["patient","ambulance"]},{"_id":ObjectId("F1178D1F969A8A7960A95091"),"address":{"current_address":{"city":"Sivpuri"}},"roles":["doctor"]},{"_id":ObjectId("8D66B95E6BCB9F70D6A05DF0"),"address":{"is_same_as_old":true},"roles":["doctor"]},{"_id":ObjectId("AB445BFCA300A4CF3CECC847"),"address":{"is_same_as_old":true},"roles":["ambulance"]}],centers:[{"_id":ObjectId("B17579ACAA5DF32543E9CC10"),"general":{"address":{"city":"Ahamdabaad"}},"center_type":"clinic","created_by":ObjectId("8B0F857017EA90AA0042C33B")},{"_id":ObjectId("F489C3147C2E380C21E1A6DB"),"general":{"address":{"city":"Devpuri"}},"center_type":"ambulance","created_by":ObjectId("5B103AF02C975AC3207AF88D")},{"_id":ObjectId("314444EA01F4BEA1444CE0F1"),"general":{"address":{"city":"Manipur"}},"center_type":"clinic","created_by":ObjectId("F1178D1F969A8A7960A95091")},{"_id":ObjectId("5D10228ABC33DFDF71EDC464"),"general":{"address":{"state":"Asam"}},"center_type":"clinic","created_by":ObjectId("8D66B95E6BCB9F70D6A05DF0")},{"_id":ObjectId("004F97892204D8F5351B1BFF"),"general":{"address":{"state":"UP"}},"center_type":"ambulance","created_by":ObjectId("AB445BFCA300A4CF3CECC847")}]}

db.user.aggregate([{"$lookup":{"from":"centers","localField":"_id","foreignField":"created_by","as":"clinics"}},{"$addFields":{clinics:{"$filter":{input:"$clinics",as:"clinic",cond:{$and:[{$eq:["$$clinic.center_type","clinic"]}]}}}}},{$addFields:{clinics_length:{$size:"$clinics"}}},{$addFields:{city:{$switch:{branches:[{case:{$ne:["clinics_length",0]},then:"$clinics.general.address.city"},{case:{$gt:["address.current_address",null]},then:"$address.current_address.city"}],default:"-"}}}},{$addFields:{city:{$cond:{if:{$and:[{$isArray:"$city"},{$ne:["$city",[]]}]},then:{$arrayElemAt:["$city",0]},else:"$city"}}}}])