Pymongo find with a projection operator

Question

I have a nested mongodb database and I am trying to perform a query which find entries and returns only certain fields.

The fields that I want returning are nested

The database looks like this

 {
 'material_type':'solid',
 'performance':10,
 'material': {'isotopes': [ { 'abundance': 0.9,
                              'atomic_number': 6,
                             },
                             { 'abundance': 0.1,
                               'atomic_number': 7,
                             }
                           ]
                },
 },
 {
 'material_type':'solid',
 'performance':9,
 'material': {'isotopes': [ { 'abundance': 0.7,
                                 'atomic_number': 6,
                             },
                             { 'abundance': 0.3,
                                 'atomic_number': 7,
                             }
                           ]
                }
 }

I would like to return the nested abundance field but only if the atomic number is equal to 6.

I have tried performing a projection on the query and currently have something like this in python pymongo

 results = database.find({'material_type':'solid'},
                         {'performance':True,
                          'material.isotopes':True 
                         })

I think that I need a projection operation but can't get them to work in pymongo. Any ideas what the pymongo database.find operation should be to return the below fields and values?

  performance , abundance 
  10              0.9
  9               0.7

Answer 1

When using a projection you need to use either 1 or 0 and not True or False respectively.

Try this:

find( {'material_type':'solid', 
      'material.isotopes.atomic_number' : {'$eq': 6 } 
      },
      {'_id' : 0, 'performance' : 1,  
      'material.isotopes.atomic_number.$' : 1 } )

Returns:

{
    "performance" : 10.0,
    "material" : {
        "isotopes" : [ 
            {
                "abundance" : 0.9,
                "atomic_number" : 6.0
            }
        ]
    }
}

/* 2 */
{
    "performance" : 9.0,
    "material" : {
        "isotopes" : [ 
            {
                "abundance" : 0.7,
                "atomic_number" : 6.0
            }
        ]
    }
}

You use $ in the projection when you only need one particular array element in selected documents. You could of tried $elemMatch if your array wasen't nested.

You could then put the results into a list and then select the two elements you want to print:

results = list( db.collection_name.find(
          {'material_type':'solid',  
          'material.isotopes.atomic_number' : {'$eq': 6 }},
          {'performance':1, 
           'material.isotopes.atomic_number.$':1 }
          ))

I'm running pymongo 3.6.0 and mongodb v3.6