QuerySet.distinct().order_by()[slice].count() crashes with "TypeError: can only concatenate tuple (not "list") to tuple"

[Stephen Brooks]

Seen in django version 2.0.2
A model query triggers line 1448 in the file django/db/models/sql/compiler.py to raise a TypeError:

params = params + self.query.sub_params

TypeError exception: can only concatenate tuple (not "list") to tuple

This line is in the as_sql method of the SQLAggregateCompiler class.

How to reproduce:
Issue a query on a model which is related to another model, and include distinct, order_by (on a string field in the related model), a slice and a call to count().

e.g.

Post.objects.distinct().order_by('author__last_name')[:20].count()

The presence of distinct() above, you could argue, is not needed, however, the above represents a simplification of the more complex query with which I originally saw the problem, and there the distinct() followed a more complex cross-table filter.

[Mariusz Felisiak]

Can you give more details about a Post and Author models? I couldn't reproduce this issue.

[Stephen Brooks]

Replying to felixxm:

Can you give more details about a Post and Author models? I couldn't reproduce this issue.

They are just sample model names. The important thing is that there is a foreign key relationship from Post to Author so that the query can do an order_by on a char field in the related model.

When trying to reproduce it, don't leave out any of: distinct, order_by(a char field in the related model), slice, and count.

[Tim Graham]

I reproduced with Book.objects.distinct().order_by('publisher__name')[:20].count() for tests/aggregation (not the right place commit the test). Bisected to 4acae21846f6212aa992763e587c7e201828d7b0.

[Simon Charette]

​https://github.com/django/django/pull/9660

[Tim Graham <timograham@…>]

In d61fe246:

Fixed #29108 -- Fixed crash in aggregation of distinct+ordered+sliced querysets.

Regression in 4acae21846f6212aa992763e587c7e201828d7b0.
Thanks Stephen Brooks for the report.

[Tim Graham <timograham@…>]

In 8d03356:

[2.0.x] Fixed #29108 -- Fixed crash in aggregation of distinct+ordered+sliced querysets.

Regression in 4acae21846f6212aa992763e587c7e201828d7b0.
Thanks Stephen Brooks for the report.

Backport of d61fe246015aa4fdc6dcb837ffb1442fa71ae586 from master