Allow Expressions when saving new models

[Alex Hill]

Opening a ticket after discussion on IRC with Josh.

You can use Expression when updating models, but it would also be useful to be able to use them when creating models, particularly in order to apply database functions to newly-inserted values.

My specific use case for this is a PostgreSQL TSVectorField, where instead of just inserting a value, I want to insert strip(to_tsvector(<ts_config>, unaccent(<value>))). Ideally, I would be able to define these all as Funcs and insert an Expression.

I propose to fix this by making SQLInsertCompiler check for resolve_expressions and as_sql, as SQLUpdateCompiler does.

Alex

Update: patch at ​https://github.com/django/django/pull/5099

[Anssi Kääriäinen]

Sounds like a good idea to me. This will also be one step towards database based default values which could be a nice feature.

[Ravi Kotecha]

I've started to look at this but if anyone knows this part of the codebase well then I think they'd be able to add this quite quickly so feel free to re-assign this if you want to work on it

[Adam Johnson]

I also have a use-case for this - I'm trying to implement MariaDB Dynamic Columns which require calling a function to create ( ​https://mariadb.com/kb/en/mariadb/dynamic-columns/#column_create ).

[Carl Meyer]

Just for future reference for whoever works on this: I spent a few minutes looking at this tonight and I think that implementing this _might_ allow us to get rid of the whole Field.get_placeholder() hook, which is effectively just a hacky way to allow fields to wrap their values in an arbitrary SQL expression at save time. (Currently only GIS fields use this hook in core.)

In the meantime, if you're wishing Django 1.8 had this feature, get_placeholder() appears to be a usable workaround (for any given specific field type; doesn't offer generic support).

[Alex Hill]

Here's my patch implementing this: ​https://github.com/django/django/pull/5099

It also adds support for expressions (and placeholders) in bulk inserts.

Notes:

• As Carl suggested, this should allow us to get rid of get_placeholder() entirely.
• The handling of AutoFields is a bit ugly at the moment. If an AutoField's value is None, it's removed from the list of fields in Model._save_table() and then conditionally re-added in SQLInsertCompiler.as_sql(). I'm thinking about nicer ways to handle this.
• There is a heap of same-but-different code between SQLInsertCompiler and SQLUpdateCompiler, especially now that expressions are supported. I'd like to clean this up in a subsequent patch.
• I've changed the workaround for #10888. I'm pretty sure it won't change anything, but I can't test in Oracle.