Removing datastore key query filter restriction.

[dhermes]

Also checking in Query constructor that all the filters are valid.

Fixes #917.

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@pcostell Can you verify that key inequality filters work? (The docs clearly state they do, just want to be careful.) This has been in the codebase longer than I've been working on the project, so maybe it was introduced before inequality filters were allowed?

[pcostell]

Yes, key inequalities are allowed.

[tseaver]

@pcostell does the API allow arbitrary rich comparisons on keys (<, <=, >, >=), or just "not equal" (!=). If ordering comparisons are allowed, how is the ordering done?

[dhermes]

@tseaver check out #917 for your answer

[tseaver]

I see that @vinays uses >= as the operator in his workaround: I'm just not sure whether / how the back-end interprets ordering comparisons on keys (as opposed to simple equality / inequality).

[pcostell]

Yes I mean <, >, <=, and >=. Like property filters, != is not supported. Ordering is performed on the key path tuples and names are always greater than ids. So
(MyParent, 123), (MyKind, "bar") > (MyParent, 123), (MyKind, 12) > (MyParent, 123), (MyKind, 6) > (MyParent, 123) > (MyParent, 122)

[tseaver]

@pcostell OK, thanks. For future reference, I found the docs for key filters which describe those semantics (but not as clearly as you did :).

[tseaver]

@dhermes LGTM