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schema-update-thoughts.md

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Schema calculation

First, here's the way that the schema calculation currently works:

  1. Each Django content type for which you want to use JSONAttributesField has a list of schema selectors configured in the Django settings. These selectors are period-separated sequences of model field names to follow to find a selector value (with pk getting you the primary key of whatever you're looking at). Some examples: project.organization.pk, project.pk, type. All but the last of the elements in these period-separated sequences should be the names of foreign key fields. The last element in the sequence can be either a "normal" field name (whose value is convertible to a string) or pk (to use the primary key of a model instance referred to via a foreign key).

  2. Each Schema model instance in the database has a content type to which it applies, a fixed sequence of schema selector values and a set of attribute fields. A Schema instance is said to be applicable to a model instance containing a JSONAttributesField field if the content types of the schema and model instances match and the schema selector list for the Schema instance is a prefix of the list of schema selectors for the model instance.

  3. To find the effective attribute list for a model instance containing a JSONAttributesField field, the attribute lists of all Schema instances that are applicable for the model instance are composed in order from least to most specific (one Schema is less specific than another if its schema selector list is shorter).

Example scenario

class B(models.Model):
  pass

class C(models.Model):
  f3 = models.ForeignKey(D)

class D(models.Model):
  pass

class A(models.Model):
  f1 = models.ForeignKey(B)
  f2 = models.ForeignKey(C)
  f4 = models.CharField(max_length=32)
  f5 = models.CharField(max_length=100)
  attrs = JSONAttributesField()

Schema selectors for content type A are (f1.pk. f2.f3.pk, f4).

Schema selector updating

  1. Change to any field in instance of A not involved in schema selectors => no schema change.
  2. Change to a.f1 => recalculate schemas.
  3. Change to a.f2 => recalculate schemas.
  4. Change to a.f2.f3 => recalculate schemas?
  5. Change to a.f4 => recalculate schemas.

Schema recalculation involves the following steps, which should run in a pre_save signal handler on any model class containing a JSONAttributesField field:

  1. Save old instance attribute list.
  2. Calculate new attribute list from new selectors.
  3. Determine the difference between the new and old attribute lists.
  4. If the differences contain any illegal attribute list changes, raise an exception (thus preventing the model instance update by cancelling the save action).
  5. If the attribute list differences are OK, allow the save to proceed, and update the model instance's attribute list.