Mongodb: How to retrieve element from array?

Question

Consider this mongodb document:

 {
"_id:"0,
"firstname":"Tom",
"profiles" : [
               {
                    "profile_name": "tom",
                    "reward_programs:[
                                        {
                                            'program_name':'American',
                                            'username':'tomdoe',
                                        },
                                        {
                                            'program_name':'Delta',
                                            'username':'tomdoe',
                                        }
                                      ]

                    "settings": {
                                    'auto_update': "False"
                                }
               },
               {
                    "profile_name": "harry",
                    "reward_programs:[
                                        {
                                            'program_name':'American',
                                            'username':'car',
                                            'account':'train',
                                        },
                                        {
                                            'program_name':'Delta',
                                            'username':'harrydoe',
                                        }
                                      ]

                    "settings": {
                                    'auto_update': "False"
                                }
               }
            ]

}

How would I retrieve just the 'settings' dictionary for a particular profile name? Let's use 'harry' in the example. I have tried:

result = users.find_one({'_id': request._id, 'profiles.profile_name': 'harry'}, {'_id': 0,  'profiles.$.settings': 1})

But this retrieves the entire dictionary of profile_name:'harry'

  {
    'profiles':
               {
                    "profile_name": "harry",
                    "reward_programs:[
                                        {
                                            'program_name':'American',
                                            'username':'car',
                                            'account':'train',
                                        },
                                        {
                                            'program_name':'Delta',
                                            'username':'harrydoe',
                                        }
                                      ]

                    "settings": {
                                    'auto_update': "False"
                                }
               }
 }

I would prefer getting the result as

 {
"profiles" : [
               {
                    "settings": {
                                    'auto_update': "False"
                                }
               }
            ]

}

And if simple enough I would even prefer:

               {
                    "settings": {
                                    'auto_update': "False"
                                }
               }

I obviously have my projection messed up but I don't know how to fix it. Suggestions?

Abhay PS · Accepted Answer · 2014-09-27 20:27:23Z

1

You need to use aggregation framework for this. Something like this will give what you want -

result = db.users.aggregate( [ 
                     { $unwind : "$profiles"}, 
                     { $match : { "profiles.profile_name" : "harry"}}, 
                     { $project : { settings : "$profiles.settings" } }
                     ] )

If you don't want '_id' field then you can hide it like this -

 result = db.users.aggregate( [ 
                         { $unwind : "$profiles"}, 
                         { $match : { "profiles.profile_name" : "harry"}}, 
                         { $project : { '_id' : 0, settings : "$profiles.settings" } }
                         ] )

The result variable will have -

{ "settings" : { "auto_update" : "False" } }

edited Sep 27, 2014 at 20:27

answered Sep 27, 2014 at 20:21

Abhay PS

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1 Comment

fat fantasma Over a year ago

I was hoping not to use the aggregation framework. But if that is the only way so be it. Do you know what would be faster, using the aggregation framework or just doing what I did and pull out 'settings' manually from the result dict?

gnub · Accepted Answer · 2015-08-28 11:23:41Z

0

I guess mongodb has added some functionality since the question was answered. Just adding an answer for being current. In the projection you could just give the key from the embedded document and get the required embedded document alone.

result = users.find_one({'_id': request._id, 'profiles.profile_name': 'harry'},{'_id': 0,'settings': 1})

answered Aug 28, 2015 at 11:23

gnub

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