Keep track of changes to fields
===============================

In some circumstances, you want to keep track of changes made on some
fields of the master model.

**django-cqrs** can send previous values of the tracked fields
to replicas.

To do so, you can use the `CQRS_TRACKED_FIELDS` attribute to specify
which fields to track:

``` py3
class MyMasterModel(MasterMixin):

    CQRS_ID = 'my_model'
    CQRS_TRACKED_FIELDS = ('char_field', 'parent', 'status')


    char_field = models.CharField(max_length=100)
    status = models.CharField(max_length=15, choices=STATUSES)

    parent = models.ForeignKey(ParentMode, on_delete=models.CASCADE)
```

This way, you can override the `cqrs_save` and apply your persistence
logic based o tracked fields before accessing your database:

``` py3
class MyReplicaModel(ReplicaMixin):

    CQRS_ID = 'my_model'

    @classmethod
    def cqrs_save(cls, master_data, previous_data=None, sync=False):
        # Custom logic based on previous_data here.
        pass
```

!!! note

    The fields tracking features honors the `CQRS_MAPPING` attribute.


!!! note

    The fields tracking features relies on the
    [FieldTracker](https://django-model-utils.readthedocs.io/en/latest/utilities.html#field-tracker)
    utility class from the
    [django-model-utils](https://github.com/jazzband/django-model-utils)
    library.