使用 Pymongo 查找数组内的文档

Question

I am really new on Mongo, I have been trying to extract a document that is inside an array, using pymongo, without any result. The next image shows the structure of the mongos database.

I just want to extract for example the first or the second document that belongs to the "data" array. I have tried with the next codes:

data = condition.find_one({},{'data.ref':2})
data = condition.find({ 'data': {'$eq': '1'} })
data = condition.find({'data.ref':1})
data = condition.find({'_id':1},{'data.ref':1})
data = condition.find({'data.ref':1},{'trq.min':50})
data = condition.find({ "data" : { "$elemMatch" : { "ref" : 1} }})

I would appreciate any reference to consult or any help in order to extract a single document from this array.

Answer 1

You can use the $elemMatch(projection) to project that desired array value.

Please note that I just convert the cursor to a list for easy handling.

Based on the following document:

{'_id': 1,
 'data': [{'1': {'trq': {'max': 100, 'min': 50}}, 'ref': 1},
          {'2': {'trq': {'max': 50, 'min': 50}}, 'ref': 2}],
 'max': 100,
 'min': 30}

The following query will return the first ref and suppress the id:

mlist1 = list(coll.find({}, {
    '_id': 0,
    'data': {'$elemMatch': {'ref': {'$eq': 1}}}
}))

Results:

{'data': [{'1': {'trq': {'max': 100, 'min': 50}}, 'ref': 1}]}

For the second one:

mlist1 = list(coll.find({}, {
    '_id': 0,
    'data': {'$elemMatch': {'ref': {'$eq': 2}}}
}))

Results:

{'data': [{'2': {'trq': {'max': 50, 'min': 50}}, 'ref': 2}]}