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Fixed #18757, #14462, #21565 -- Reworked database-python type conversions

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Complete rework of translating data values from database

Deprecation of SubfieldBase, removal of resolve_columns and
convert_values in favour of a more general converter based approach and
public API Field.from_db_value(). Now works seamlessly with aggregation,
.values() and raw queries.

Thanks to akaariai in particular for extensive advice and inspiration,
also to shaib, manfre and timograham for their reviews.
This commit is contained in:
Marc Tamlyn
2014-08-12 13:08:40 +01:00
parent 89559bcfb0
commit e9103402c0
35 changed files with 443 additions and 521 deletions
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54
docs/ref/models/fields.txt
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@@ -1532,7 +1532,7 @@ Field API reference
``Field`` is an abstract class that represents a database table column.
Django uses fields to create the database table (:meth:`db_type`), to map
Python types to database (:meth:`get_prep_value`) and vice-versa
(:meth:`to_python`), and to apply :doc:`/ref/models/lookups`
(:meth:`from_db_value`), and to apply :doc:`/ref/models/lookups`
(:meth:`get_prep_lookup`).
A field is thus a fundamental piece in different Django APIs, notably,
@@ -1609,17 +1609,26 @@ Field API reference
See :ref:`converting-query-values-to-database-values` for usage.
When loading data, :meth:`to_python` is used:
When loading data, :meth:`from_db_value` is used:
.. method:: to_python(value)
.. method:: from_db_value(value, connection)
Converts a value as returned by the database (or a serializer) to a
Python object. It is the reverse of :meth:`get_prep_value`.
.. versionadded:: 1.8
The default implementation returns ``value``, which is the common case
when the database backend already returns the correct Python type.
Converts a value as returned by the database to a Python object. It is
the reverse of :meth:`get_prep_value`.
See :ref:`converting-database-values-to-python-objects` for usage.
This method is not used for most built-in fields as the database
backend already returns the correct Python type, or the backend itself
does the conversion.
See :ref:`converting-values-to-python-objects` for usage.
.. note::
For performance reasons, ``from_db_value`` is not implemented as a
no-op on fields which do not require it (all Django fields).
Consequently you may not call ``super`` in your definition.
When saving, :meth:`pre_save` and :meth:`get_db_prep_save` are used:
@@ -1644,15 +1653,6 @@ Field API reference
See :ref:`preprocessing-values-before-saving` for usage.
Besides saving to the database, the field also needs to know how to
serialize its value (inverse of :meth:`to_python`):
.. method:: value_to_string(obj)
Converts ``obj`` to a string. Used to serialize the value of the field.
See :ref:`converting-model-field-to-serialization` for usage.
When a lookup is used on a field, the value may need to be "prepared".
Django exposes two methods for this:
@@ -1682,6 +1682,26 @@ Field API reference
``prepared`` describes whether the value has already been prepared with
:meth:`get_prep_lookup`.
Fields often receive their values as a different type, either from
serialization or from forms.
.. method:: to_python(value)
Converts the value into the correct Python object. It acts as the
reverse of :meth:`value_to_string`, and is also called in
:meth:`~django.db.models.Model.clean`.
See :ref:`converting-values-to-python-objects` for usage.
Besides saving to the database, the field also needs to know how to
serialize its value:
.. method:: value_to_string(obj)
Converts ``obj`` to a string. Used to serialize the value of the field.
See :ref:`converting-model-field-to-serialization` for usage.
When using :class:`model forms <django.forms.ModelForm>`, the ``Field``
needs to know which form field it should be represented by: