mongodb findAndModify update element in array -

there bson document:

{     "_id" : objectid("5718441f5116a60b08000b8c"),     "mails" : [         {             "id" : 2,             "a" : [                 {                     "a" : 1                 },                 {                     "a" : 2                 }             ]         },         {             "id" : 1,             "a" : [                 {                     "a" : 1                 },                 {                     "a" : 2                 }             ]         }     ] } 

i need return , clear array "a" belong "mails.id == x" given document. use findandmodify like:

db.mail.findandmodify({query: {"_id":objectid("5718441f5116a60b08000b8c")}, update: {$set:{"mails.$.a":[]}}, new: false, fields:{"mails":{$elemmatch:{"id":1}}}}) 

however don't work. problem $set should apply on 1 document in array rather whole document. need projection project out.

if left update blank, return desired part:

{     "_id" : objectid("5718441f5116a60b08000b8c"),     "mails" : [         {             "id" : 1,             "a" : [                 {                     "a" : 1                 },                 {                     "a" : 2                 }             ]         }     ] } 

but don't know how clear array 'a' in 'mails'

you have specify array element match in query:

db.mail.findandmodify({query: {"_id":objectid("5718441f5116a60b08000b8c"), "mails":{$elemmatch:{"id":1}}}, update: {$set:{"mails.$.a":[]}}, new: false, fields:{"mails":{$elemmatch:{"id":1}}}})

mails.$ in update matches first matched element in doc, have match in query. also, query update doc, return old version, since use new: false, if want updated version set true.