Here is a list of issues to solve to support explicit transforms and
lookups by filter (and exclude) methods.
1. Make Lookup.__init__ signature to support initialization with F objects
or string path (e.g. GreaterThan(F('user__id'), 10) or
GreaterThan('user__id', 10)), not sure it's possible to use simultaneously
with the current approach with lhs, rhs initialization (even with moving it
to a separate class method, e.g Lookup.build(lhs, rhs)), so I assume
creating so-called util classes which will delegate SQL-related
functionality to existing Lookup classes.
2. Chain transforms by passing them as argument:
Lower(Unaccent(F('user__name)))
3. Decide if Q objects shall support explicit lookups/transforms as
argument as well - it's a kind of logical step, as without Q objects it
will not be possible to perform complicated conditions (AND, OR, NOT).
In that case lookup/transform parsing should be moved from QuerySet object
to Q object - filter will take already parsed lookup tree.
Example:
Q(user__name__lower__unaccent__icontains='Bob') will internally parse it
and build next structure:
Q(Icontains(Lower(Unaccent(F('user__name')))), 'Bob')
On Sunday, August 16, 2015 at 4:18:26 PM UTC+3, Alexey Zankevich wrote:
>
> Hi all,
>
> This topic is related to the current ORM query syntax with underscores.
> There are lots of arguing related to it, anyway it has pros and cons.
>
> Let's take a concrete example of querying a model:
>
> >>>
> GameSession.objects.filter(user__profile__last_login_date__gte=yesterday)
>
>
> Pros:
>
> 1. The syntax is easy to understand
> 2. Can be extended with custom transforms and lookups
>
> However, there are several cons:
>
> 1. Long strings is hard to read, especially if we have fields with
> underscores.
> It's really easy to make a mistake by missing one:
>
> >>>
> GameSession.objects.filter(user_profile__last_login_date__gte=yesterday)
>
> Not easy to catch missing underscore between user and profile, is it? Even
> though, it's not easy to say whether it should be "user_profile" attribute
> or
> user.profile foreign key.
>
> 2. Query strings can't be reused, thus the approach violates DRY principle.
> For example, we need to order results by last_login_date:
>
> >>>
> GameSession.objects.filter(user__profile__last_login_date__gte=yesterday) \
> .order_by('user__profile__last_login_date')
>
> We can't keep user__profile_login_date as a variable as in the first part
> of the
> expression we use a keyword argument, meanwhile in the second part - just
> a
> string. And thus we just have to type query path twice.
>
> 3. Lookup names not natural to Python language and require to be
> remembered or
> looked up in documentation. For example, "__gte" or "__lte" lookups tend
> to be
> confused with "ge" and "le" due to similarity to methods "__ge__" and
> "__le__".
>
> 4. Lookup keywords limited to a single argument only, very inconvenient
> when
> necessary to filter objects by range.
>
> I was thinking a lot trying to solve those issues, keeping in mind Django
> approaches. Finally I came up with solution to extend Q objects with dot
> expression syntax:
>
> >>> GameSession.objecs.filter(Q.user.profile.last_login_date >= yesterday)
>
> Q is a factory instance for old-style Q objects. Accessing attribute by dot
> returns a child factory, calling factory will instantiate old-style Q
> object.
>
> >>> Q
> <QFactory object at 0x7f407298ee10>
>
> >>> Q.user.profile
> <QFactory object at 0x7f40765da310>
>
> >>> Q(user__name='Bob')
> <Q: (AND: ('user__name', 'Bob'))>
>
> It overrides operators, so comparing factory with value returns a related Q
> object:
>
> >>> Q.user.name == 'Bob'
> <Q: (AND: ('user__name', 'Bob'))>
>
> Factory has several helper functions for lookups which aren't related to
> any
> Python operators directly:
>
> >>> Q.user.name.icontains('Bob')
> <Q: (AND: ('user__name__icontains', 'Bob'))>
>
> And helper to get query path as string, which requred by order_by or
> select_related queryset methods:
>
> >>> Q.user.profile.last_login_date.get_path()
> 'user__profile__last_login_date'
>
> You can check implementation and more examples here
> https://github.com/Nepherhotep/django-orm-sugar
>
> How it solves issues:
>
> #1. Dots hard to confuse with underscores
> #2. Query paths can be reused:
>
> >>> factory = Q.user.profile.last_login_date
> >>> query = GameSession.objects.filter(factory >= yesterday)
> >>> query = query.order_by(factory.get_path())
>
> #3. Not neccessary to remember most of lookup names and use comparison
> operators
> instead.
> #4. Possible to use multiple keyword arguments:
>
> >>> Q.user.profile.last_login_date.in_range(from_date, to_date)
> <Q: (AND: ('user__profile__last_login_date__lte', from_date),
> ('user__profile__last_login_date__gte', to_date))>
>
>
> This approach looked the best for me due to several reasons:
>
> 1. It's explicit - it doesn't do anything but generating appropriate Q
> object.
> The result of comparison can be saved as Q object variable.
>
> 2. It's short - variants with using model for that will look much longer,
> when
> joining two or more filters:
>
> >>> GameSession.objects.user.profile_last_login_date >= yesterday #
> awkward
>
> 3. Implementation will not require to change querset manager or model
> classes
>
> 4. Will still allow to use filters and Q class in the old way:
>
> >>> q = Q(user__profile__last_login_date__gte=yesterday)
>
> or
>
> >>>
> GameSession.objects.filter(user__profile__last_login_date__gte=yesterday)
>
> I'd like to make it as a part of Django ORM syntax and it will not be hard
> to
> do, especially taking into account the library is already done and
> working.
> Anyway, I need your thought about the idea in general, as well as about
> particular things like chosen method names - "get_path", "in_range" and
> etc.
> As next step I can create a ticket in the issue tracker, or prepare DEP
> first.
> In latter case I need to find a shepherd to work with.
>
> Best regards,
> Alexey
>
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