Tortoise-orm: Add database functions in queries to support manipulation of fields

[zoliszeredi]

Is your feature request related to a problem? Please describe.
As of v0.13.2 there is no support for database functions. Depending on your use-case, query fields might require extra processing:

• for NULLs COALESCE for comparisons and conversions
• for text types: TRIM for trailing white-spaces, LENGTH, REPLACE, SUBSTRING, CONCAT

Describe the solution you'd like
Add support for functions via an importable module (e.g. tortoise.models.functions)
that implements database functions (like COALESCE) and can wrap model fields possibly via annotations like:

from tortoise.models.functions import Coalesce

class User(Model):
    id = fields.IntField(pk=True)
    name = fields.TextField()
    email = fields.TextField()
    username = fields.TextField()
    join_date = fields.DateTimeField()

users = await User.annotate(
    screen=Coalesce("name", "username", "email"),
    clean_join_date=Coalesce('join_date', datetime.datetime(2011, 12, 31),
).filter(
    clean_join_date__gt=datetime.datetime(2012, 1, 1)
).values_list('screen', 'clean_join_date')

Describe alternatives you've considered
The two most well known ORMs for python SQLAlchemy and Django ORM have similar APIs:

Additional context
The underpinning SQL library pypika does implement and have documented the most important database functions.

From the pypika documentation for example of string functions:

from pypika import functions as fn

customers = Tables('customers')
q = Query.from_(customers).select(
    customers.id,
    fn.Concat(customers.fname, ' ', customers.lname).as_('full_name'),
)

[grigi]

Thank you for this well presented/thought out feature request @zoliszeredi

We already have support for some aggregate functions, and I don't see much new required to support other functions.
(Although the name "Aggregate" is too specific, we should probably change it to "Function")

Looking at the code, adding single-parameter functions will be trivial, multi-parameter functions will need a refactor. Shouldn't be too difficult.

Do you want to have a go?

[zoliszeredi]

Out of the myriad of functions the one that I really need is COALESCE to do easier comparison for numerical, date types when these are incomplete, missing(e.g. NULL).
I'd like to how far I can go for simpler functions by changing as you suggested Aggregate to the more generic Function.