MongoDB: Query a key having space in its name

问题:

I want to retrieve values of only certain keys from a MongoDB collection.

But, the collection has some keys which have a 'space' in their name like:

"Parent":{"key1": //some string,
          "key2": //some string,
          "key 3": //some string}

I know this is a wrong approach as there shouldn't ideally be spaces in a key name but nevertheless how do I query this key? I am using Python and PyMongo.

For normal keys I can do this:

db.coll_name.find({"key": "India"}, {"_id": 0, "Parent.key1": 1, "Parent.key2": 1})

So how do I use the key "Parent['key 3']" in the second argument of the above query? Is there any way to achieve this?

Here's the query which returns data(works):

db.coll_name.find({}, {"Parent.key1": 1, "_id": 0})

Here's the query which doesn't return data:

db.coll_name.find({}, {"Parent['key 3']": 1, "_id": 0})

回答1:

Well the only way you could have constructed this is like:

content = {};
content["Parent"] = {}
content["Parent"]["key2"] = 1
content["Parent"]["key 3"] = 1

db.coll_name.insert(content)

But you seem to be missing that there is nothing wrong with doing this:

db.coll_name.find({ "Parent.key 3":  1} )

Or in projection

 db.coll_name.find({}, { "Parent.key 3": 1 })

It's "dot notation" and not object notation, and as long as you quote the key names ( which is mandatory for dot notation ) then all it fine and you can have a space in there.

回答2:

I know this is a wrong approach as there shouldn't ideally be spaces in a key name but nevertheless how do I query this key?

What I will suggest is:

• Remove space from documents key using bulk write operations

  bulk = coll_name.initialize_unordered_bulk_op()
  count = 1000
  
  for doc in coll_name.find():
      parent = {}
      parent.setdefault('Parent', {})
      for key, val in doc['Parent'].items():
          parent['Parent'][key.replace(' ', '')] = val
          bulk.find({'_id': doc['_id']}).update({'$set': parent})
          count += 1
          if count % 1000 == 0:
              # Execute per 1000 operations and re-init.
              bulk.execute()
              bulk = coll_name.initialize_unordered_bulk_op()
  # Clean up queues
  if count % 1000 != 0:
      bulk.execute()
  

• Then your projection become simpler

  db.coll_name.find({'key': 'India'}, {'_id': 0, 'Parent.key1': 1, 'Parent.key2': 1, 'Parent.key3': 1 })