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The new Options API proposal

Official Pull Request: https://github.com/django/django/pull/2894

As of my 2014 Summer of Code project, my second deliverable is a refactored working implementation of the Options API. The Options API is at the core of Django, it enables introspection of Django Models with the rest of the system. This enables lookups, queries, forms, admin to understand the capabilities of every model. The Options API is hidden under the _meta attribute of each model class. Options has always been a private API, but Django developers have always been using it in their projects in a non-official way. This is obviously very dangerous because, as there is no official API, Options could change breaking other people's implementation. Options also did not have any unit-tests, but the entire system uses it and relies on it to work correctly.

My Summer of Code project is all about understanding and refactoring Options to make it a testable and official API that Django and any other developers can use.

Current state of the API

I now have a working and tested implementation of Options, I have managed to reduce it to 2 main endpoints. Because Options needs to be very fast, I necessarily had to add some accessors for the most common calls (although both endpoints are cached, we can increase speed by avoiding function calls). Each accessor is a cached property and is computed, using the new API, on first access.

I am planning to release in the attached PR:

Unit tests for the new Meta API
The new Meta API
The implementation of the new API throughout django and django.contrib
Documentation

Concepts

Field types

There are 5 main types of fields:

Data fields

A data field is any field that has an entry on the database, for example a CharField, BooleanField, a ForeignKey

class Person(models.Model):
    data_abstract = models.CharField(max_length=10)

M2M fields

A M2M field that is defined on the current model

class Person(models.Model):
    friends = models.ManyToManyField('self', related_name='friends', symmetrical=True)

Related Object

A Related Object is a one-to-many relation from another model (such as a ForeignKey) that points to the current model

class City(models.Model):
    name = models.CharField(max_length=100)

class Person(models.Model):
    # M2M fields
    city = models.ForeignKey(City)

In this case, City has a related object from Person

Related M2M

A Related M2M is a M2M relation from another model that points to the current model

class City(models.Model):
    name = models.CharField(max_length=100)

class Person(models.Model):
    # M2M fields
    cities_lived_in = models.ManyToManyField(City)

In this case, City has a related m2m from Person

Virtual

Virtual fields do not necessarily have an entry on the database, they are "Django fields" such as a GenericForeignKey

class Person(models.Model):
    content_type = models.ForeignKey(ContentType, related_name='+')
    object_id_ = models.PositiveIntegerField()
    item = GenericForeignKey('content_type', 'object_id')

GenericForeignKey uses 'content_type' and 'object_id' to keep track of what model type and id is set to item, but item itself does not have a concrete presence on the database. In this case, item is a virtual field.

Field options

There are 5 properties that each field can have:

Local

A local field is when is not derived from inheritance. Fields from models that directly inherit from abstract models or proxy classes are still local

class Person(models.Model):
  name = models.CharField(max_length=50)

class Londoner(Person):
  overdraft = models.DecimalField()

Londoner has two fields (name and overdraft) but only one local field (overdraft)

Hidden

Hidden fields are only referred to related objects and related m2m. When a relational model (such as ManyToManyField, or ForeignKey) specifies a related_name that starts with a "+", it tells Django to not create a reverse relation.

class City(models.Model):
    name = models.CharField(max_length=100)

class Person(models.Model):
    city = models.ForeignKey(City, related_name='+')

City has a related hidden object from Person (as you can't access person_set)

Concrete

Concrete fields are fields that have a column

Proxied relations

Proxied relations are relations that point to a proxy of a model.

class Person(models.Model):
    pass

class ProxyPerson(Person):
    class Meta:
        proxy = True

class RelationToProxy(models.Model):
     proxy_person = models.ForeignKey(ProxyPerson)

In this case, Person has no related objects, but it has 1 proxied related object from RelationToProxy.

The new API

The new API is composed of 2 main functions: get_fields, and get_field.

get_fields

    def get_fields(self, m2m=False, data=True, related_m2m=False, related_objects=False, virtual=False,
                       include_parents=True, include_non_concrete=True, include_hidden=False, include_proxy=False, export_map=False):

get_fields takes a set of flags as parameters, and returns a tuple of field instances. All possible combinations of options are possible here, although some will have no effect (such as include_proxy combined with data or m2m by itself). get_fields is internally cached for speed and it is a recursive function that collects fields from each parent of the model. An example of every (sane) combination of flags will be available in the model_meta test suite that I will ship with the new API. The 'export_map' key is only used internally (by get_field) and is not part of the public API. 'export_map=True' will return an OrderedDict with fields as keys and a tuple of strings as values. While the keys map exactly to the same output as 'export_map=False', the tuple of values will contain all possible lookup names for that field. This is used to build a fast lookup table for get_field and to avoid re-iterating over every field to pull out every possible name.

    >>> User._meta.get_fields() # Only data by default
    (<django.db.models.fields.AutoField: id>,
     <django.db.models.fields.CharField: password>,
     <django.db.models.fields.DateTimeField: last_login>,
     <django.db.models.fields.BooleanField: is_superuser>,
     <django.db.models.fields.CharField: username>,
     <django.db.models.fields.CharField: first_name>,
     <django.db.models.fields.CharField: last_name>,
     <django.db.models.fields.EmailField: email>,
     <django.db.models.fields.BooleanField: is_staff>,
     <django.db.models.fields.BooleanField: is_active>,
     <django.db.models.fields.DateTimeField: date_joined>)

    >>> User._meta.get_fields(data=False, related_objects=True) # only related_objects
    (<RelatedObject: admin:logentry related to user>,)

    >>> User._meta.get_fields(data=False, related_objects=True
                                  include_hidden=True) # only related_objects including hidden
    (<RelatedObject: auth:user_groups related to user>,
     <RelatedObject: auth:user_user_permissions related to user>,
     <RelatedObject: admin:logentry related to user>)

get_field

    def get_field(self, field_name, m2m=True, data=True, related_m2m=False, related_objects=False, virtual=False)

'get_field' returns a field_instance from a given field name. field_name can be anything from name, attname and related_query_name. get_field is recursive by default and does not include any hidden or proxied relations. If a given name is not found, it will raise a FieldDoesNotExist error. 'get_field' is internally cached and gets all field information from 'get_fields' internally.

NOTE: There is an inconsistency between the defaults of get_field and get_fields. 'get_fields' by default enables only data fields while 'get_field' by default enables data and m2m. This is because of backwards-compatibility issues (read more below).

    >>> User._meta.get_field('username') # A data field
    <django.db.models.fields.CharField: username>

    >>> User._meta.get_field('logentry', related_objects=True) # A related object
    <RelatedObject: admin:logentry related to user>

    >>> LogEntry._meta.get_field('user') # ForeignKey can be queried by field name
    <django.db.models.fields.related.ForeignKey: user>
    >>> LogEntry._meta.get_field('user_id') # .. and also by database column name
    <django.db.models.fields.related.ForeignKey: user>

    >>> User._meta.get_field('does_not_exist') # A non existent field
    *** FieldDoesNotExist: User has no field named 'does_not_exist'

The Decision Process

Since I started my Summer of Code project, this API has gone through several designs, and has now finalised onto the one shown above. The API has gone through many transformations. Each decision has gone through my mentor, with whom I have weekly meetings (Russell).

Using bitfields as flags

get_field and get_fields were originally designed to work with bits. The main choice for this decision was because there were many options and to avoid providing too many flags. The original API for bits is:

    DATA = 0b00001
    M2M = 0b00010
    RELATED_OBJECTS = 0b00100
    RELATED_M2M = 0b01000
    VIRTUAL = 0b10000

    # Aggregates
    NON_RELATED_FIELDS = DATA | M2M | VIRTUAL
    ALL = RELATED_M2M | RELATED_OBJECTS | M2M | DATA | VIRTUAL

    NONE = 0b0000
    LOCAL_ONLY = 0b0001
    CONCRETE = 0b0010
    INCLUDE_HIDDEN = 0b0100
    INCLUDE_PROXY = 0b1000

    def get_fields(types, opts)

There are numerous reasons why we backed away from this design: 1) There is always a need to import flags from models/options, this can bring to circular dependencies 2) Importing flags all the time can also be a nuinsance 2) Importing flags is not Pythonic at all

The decision taken was to port 'get_field' and 'get_fields' to flags. A port of the old implementation still lies here if you are interested: https://github.com/PirosB3/django/blob/soc2014_meta_refactor_upgrade/django/db/models/options.py

Removed direct, m2m, model

In the previous API, it was a common pattern to return model, direct (bool), m2m (bool). I soon realized that not only these three paramenters can be easily derived from a field_instance, but there were very few places that actually used some of the attributes (there is only 1 place where m2m is used).

The decision taken was to drop direct, m2m, model in the return type and only keep field_instance. All the rest will be derived if needed.

Removed all calls "with_model"

As said previously, it is redundant to include any model as this can be derived.

Removed the need of multiple maps

The previous implementation relied on many different cache maps internally. This is necessary, but tends to increase bug-risk when cache-expiry happens. For this reason, my implementation relies on only 2 cache tables, and I have added a specific function to do cache expiry easily (_expire_cache). The downsides of this aspect is that we cache a bit more naively (there are less layers of caching) but benchmark shows no real decrease of performance.

Used internal caching instead of lru_cache

Our first approach to caching was to use 'functools.lru_cache'. 'lru_cache' is a simple decorator that provides cache and an expiry function built-it. It worked correctly with the new API but cProfile quickly showed how a lot of computing time was done inside lru_cache itself.

The decision taken was to drop 'lru_cache' in favour of a simpler caching strategy. This is also because we really don't need the lru part of 'lru_caching'. there are only a finite number of combinations that can be called.

Use cached_properties when possible

Function calls are expensive in Python, All sensible attributes with no arguments have been transformed into cached_properties. A cached property is a read-only property that is calculated on demand and automatically cached. If the value has already been calculated, the cached value is returned. Cached properties avoid a new stack and are used for fast-access to fields, concrete_fields, local_concrete_fields, many_to_many, field_names

enabled m2m fields by default on get_field

The old get_field API was defined as follows:

    def get_fields(self, field_name, many_to_many=True)

Our first iteration of the API was to refactor this as

    def get_fields(self, field_name, include_related=True)

This was done for 2 reasons:

1) We managed to squash 2 functions (get_field and get_field_by_name) in 1 single call.
2) I could not find any reason for the many_to_many flag to exist! there can never be data and m2m fields with the same name. So this looked like a legacy parameter that was never removed (because turning it off did not break any tests).

The reason the many_to_many flag existed was for a special validation case that was not documented anywhere. Russell helped me in looking for edge cases and finally I came up with a failing test case: https://github.com/django/django/pull/2893. The test case would fail on the new API but succeed on master.

Our final iteration was to add all the field types as flags to get_field. By making m2m as first parameter, we avoid breaking existing implementations and maintain a similarity with the 'get_fields' API.

Performance

Throughout my project I have always kept an eye on performance. I have always looked for bottlenecks using cProfile and other benchmarking tools. I am happy to say no major decrease in speed has happened, actually the new implementation does a couple of optimizations that were not present in the old system. Said this, I prefer to not comment on performance but just show the benchmarks. It will be the core team to decide if this is feasible or not.

Main optimization points

Compute inverse relation map on first access

In order to find related objects, the current implementation does the following:

    for each model in apps
       for each field in model
          if field is a related object:
             if field is related to self:
                 add to related_objects

REF: https://github.com/django/django/blob/master/django/db/models/options.py#L488

This tends to be expensive, it results in a O(models * fields) complexity. We can increase performance by computing an inverse relation map on first access. This is done only once, not once per model (https://github.com/PirosB3/django/blob/soc2014_meta_refactor_upgrade_flags_get_field/django/apps/registry.py#L176).

In this way we have a map of { model : [related_object, related_object, ..] } and computing a hash lookup is O(1) (https://github.com/PirosB3/django/blob/soc2014_meta_refactor_upgrade_flags_get_field/django/db/models/options.py#L423).

Benchmarks

Here is a benchmark results table. It is benchmarking soc2014_meta_refactor_upgrade_flags_get_field (68dc11708eb2170540729b71db6bcaf4c46d6504) against django/master.

Djangobench: median of 2000 trials. https://gist.github.com/PirosB3/35a9231ee0214427321d

Backwards compatibility

All previous _meta functions will be backwards-compatible, with a DeprecationWarning.

How can YOU help

Give me as much feedback as you can, you can also ping me on IRC or mail (pirosb3 at gmail). A simple way to give feedback is also to have a look at the Pull Request and comment if you see something that can be improved.

Further API improvements

After a number of iterations, and many discussions on the Mailing List ( https://groups.google.com/forum/#!topic/django-developers/XfMUjK59Sls) we have come up with the new, simplified, API.

Model._meta API functions

    def get_fields(self, forward=True, reverse=False)

We have made a distinction between fields that are on the model (forward) and fields that point to the model (reverse). Compared to the previous API we are not making a distinction on if a field is a M2M, Foreign Key, or normal "data" field. This allows greater flexibility in the future and only makes one clear separation of field types. API consumers will be able to do further filtering using the field_flags (see below)

   def get_field(self, field_name)

We have dropped the include_relations flag because it is no longer required. The API is still 100% backwards compatible. A field can be a reverse or a forward field and can be accessed using 1 or more names (ex. {"user", "user_id"}).

Model._meta API accessors

_meta.fields

Returns all forward fields that may have relations and store only 1 single value.

_meta.many_to_many

Returns all forward fields that have relations and store multiple values.

_meta.concrete_fields

Returns all forward concrete fields that may have relations and store only 1 single value.

_meta.local_concrete_fields

Returns all forward concrete fields, defined on the current model, that may have relations and store only 1 single value.

_meta.related_objects

Returns all the reverse objects pointing to the current model.

_meta.field_names

This API has remains the same, but the newer implementation is more efficient.

Field flags

Each field and/or object defines a set of flags that are used by the developer to manually narrow down the search or iteration of the above API calls. By defining field flags, we give the possibility for developers to create custom-made field instances that can behave correctly in an admin interface or in auto-generated model forms.

field.concrete

A concrete field has a database column associated to it's field instance.

field.editable

An editable field is a field that's data can be manipulated (an Autofield for example should not be manipulated and responds False to an "editable" query).

field.has_many_values

A field that can contain 1 or more values. This does not need to necessary be a ManyToMany field, but can also be a future ArrayField.

field.has_relation

A field has a relation when it depends on 1 or more models, and therefore might generate reverse relations on the pointed model.

object.is_related_object and field.is_related_object

A very simple way to query if an object returned from get_field() or get_fields() is a reverse object or a forward object. This avoids imports and the use of 'isinstance', that is expensive.

Last modified 10 years ago Last modified on Sep 11, 2014, 9:00:50 AM

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