ModelChoiceField does not handle MultipleObjectsReturned

[Keryn Knight]

As far as I can gather [that is, this hasn't actually bitten me], the ​to_python for ModelChoiceField does not catch the queryset model's MultipleObjectsReturned exception, which in most cases will be fine because the query will be against the pk, which is ostensibly unique unless a user has gone out of their way to break things.

However, given the ability to use to_field_name (which was recently documented [in #21860], increasing the likelihood of it's usage) it is feasible that a user would end up using a field which is not unique (or even spans a relationship?), and thus in a simple key:value lookup could return multiple rows.

This would also technically affect the ModelMultipleChoiceField, whose superclass is ModelChoiceField and whose to_python method is used for each value.

[Tim Graham]

Shouldn't we just document that to_field_name must be able to uniquely identify the objects? I'm not sure what error message we'd present since it's presumably a programming error and not something a user could fix.

[Tim Graham <timograham@…>]

In 83762da3fbc00fdb5c6cfbcd04e25ff00c599102:

Fixed #23354 -- Documented that ModelChoiceField to_field_name should be a unique field.

Thanks Keryn Knight for the suggestion.

[Tim Graham <timograham@…>]

In e86030b3be2ac9af4a2855d88802c0b750152c8a:

[1.7.x] Fixed #23354 -- Documented that ModelChoiceField to_field_name should be a unique field.

Thanks Keryn Knight for the suggestion.

Backport of 83762da3fb from master