The default for ArrayAgg should be an empty list rather than None

[Markus Amalthea Magnuson]

I would expect (in Python, broadly) a function that returns a list of values to always return a list. What is the reasoning behind this not being the case for ArrayAgg? Instead, the default value is None when there are no values, and I'm sure there is a good reason for this.

[Jacob Walls]

See #10929 and fee87345967b3d917b618533585076cbfa43451b: Django used to do that, but deprecated it and removed it in 5.0, due to it being inconsistent with other aggregates. (The inconsistency was a compromise given the lack of default at the time.)

You can use default=Value([]) if you prefer. See ​https://docs.djangoproject.com/en/5.2/releases/4.0/#id2.

[Simon Charette]

You can use default=Value([]) if you prefer.

Small FYI that you don't have to do the Value wrapping for ArrayField expressions, passing default=[] works just fine.

Value wrapping is only necessary when passing a str literal as they default to field references ('field' is equivalent to F('field')) instead of value literals like any other Python literal does. That's the reason why Value wrapping is necessary for JSONBAgg and StringAgg though.

[Markus Amalthea Magnuson]

Why is [] safe to use here (rather than list)?

[Jacob Walls]

Django never mutates the value; it just sends it to the db adapter to include in the query.

[Simon Charette]

In other words, the ARRAY_AGG Postgres function returns NULL ​on an empty set.

Passing default=[] wraps the ARRAY_AGG in COALESCE to default to the provided value

SELECT COALESCE(ARRAY_AGG(field), '{}'::text[])

Per Postgres documentation linked above

It should be noted that except for count, these functions return a null value when no rows are selected. In particular, sum of no rows returns null, not zero as one might expect, and array_agg returns null rather than an empty array when there are no input rows. The coalesce function can be used to substitute zero or an empty array for null when necessary.

Concretely this means that Aggregate(default) is a nicer way to express Coalesce(Aggregate(...), default).