MongoDB - 对不存在的字段进行错误处理

Question

I have a collection with a set of field values for each document. One of these is called 'coordinates'. Now when I'm querying the DB for and for elements that are not null for this field, it returns the right values as I'm expecting.

However I run into this error now and then in python (Pymongo):

if not doc['coordinates']
TypeError: 'NoneType' object has no attribute '__getitem__'

This seems to imply that it has come across a record where there is no field 'coordinates'. I've created the document to have this field and should exist.

I want to know though, how can I handle this error and to prevent this error from terminating my program.

This is how I find the appropriate queries:

 cursor = collection.find(
                { "$and" : [
                    {"term": {"$in": events}}, 
                    { "$or" : [
                            {"coordinates" : {"$ne": None}}, 
                            {"place" : {"$ne" : None}}
                    ]}
                ]}, 
            {"term" : 1, "coordinates" : 1, "place" : 1, "time_normal" : 1}, tailable = True, timeout = False )

I then iterate through the returned queries, as:

while cursor.alive:
    try:
        doc = cursor.next()

        CODE IS HERE TO QUERY DB


    except StopIteration:
        pass

Thanks

Answer 1

The reason you're getting a TypeError in your example and an AttributeError in the example that nickmilon supplied is because the query that you're using to get your doc is not finding anything (and is therefore a NoneType ). Check your query to make sure that you're actually getting things out of your collection.

The answer that nickmilon supplies should give you the error handling you're after, though.

Answer 2

您可以尝试：doc.get（'coordinates'）如果字段不存在则返回None，如果存在则返回其值

Answer 3

Eric is right, for some reason your doc is None. Could you post here the code you are using for find and how you process the cursor returned from find ?

Answer 4

I was able to handle this error by including the TypeError in the exception block, as:

                try:                                               
                    if not doc['coordinates']:
                        CODE HERE

                    else: 
                        CODE HERE

                except (TypeError):
                    pass