Google App Engine（python）：确定查询是否返回结果的最佳方法

Question

I'm using the webapp framework on GAE, and to show the results of a query, I'm doing a get() on the query object, then iterating on it if get() returned anything, for example:

query = Employee.all().filter("some_boolean_property = ", True)
if query.get():
    for employee in query:
        # output employee.name etc.
        # ...
else:
    # output "no records found" message
    # ...

The reason I'm doing the get() and not just doing an else on the for loop is because I'm outputting the data in a table, and I don't want to write the table code if there are no results. Previously instead of the get() I was doing fetch(1), but I believe they are equivalent (ie, get() just executes the query but with a maximum of one result). Therein lies my question - is this true that I can use get() in this way, and is this the best way to do find out if a query returns results or not? Might count(1) be better?

I'm not concerned with the number of results, just if there are any or not.

Answer 1

Just call fetch to get the number of results you need, then iterate over them. For example:

query = Employee.all().filter("some_boolean_property = ", True)
results = query.fetch(20)
if results:
    for employee in results:
        # Do stuff

Answer 2

An alternative might be to set a flag inside the loop to say that at least one record is found, then test that flag for your "no records found" case.

found = False
query = Employee.all().filter("some_boolean_property = ", True)
for employee in query:
    found = True
    # output employee.name etc.
    # ...
if not found:
    # output "no records found" message
    # ...

This has the advantage of removing a call to the datastore.

Answer 3

You can use the Query class's count method. If you provide a limit it will only check that many. Here's an example:

query = Employee.all().filter("some_boolean_property = ", True)
if query.count(limit=1):
   for employee in query:
       pass

Answer 4

'count' returns just the number of results found with the query

'get' and 'fetch' return the entire entity

so the 'count' is much more efficient if you only want the count of a query

Also I dont see why people are running a loop after a query for 1 item. If you use get() you will only receive 1 item that matches so you dont need a loop. All you are really checking is if you actual got an entity or count back from the query.