GlitchTip/psqlextra/backend/migrations/README.md

What's up with the shady patch functions?

Django currently does not provide a way to extend certain classes that are used when auto-generating migrations using makemigrations. The patch functions use Python's standard mocking framework to direct certain functions to a custom implementation.

These patches allow django-postgres-extra to let Django auto-generate migrations for PostgresPartitionedModel, PostgresViewModel and PostgresMaterializedView.

None of the patches fundamentally change how Django work. They let Django do most of the work and only customize for Postgres specific models. All of the patches call the original implementation and then patch the results instead of copying the entire implementation.

Using the patches

The patches are all context managers. The top level postgres_patched_migrations context manager applies all patches for the duration of the context.

This is used in the custom pgmakemigrations command to extend the migration autodetector for PostgresPartitionedModel, PostgresViewModel and PostgresMaterializedView.

Patches

Autodetector patch

• Patches django.db.migrations.autodetector.MigrationAutodetector.add_operation

This function is called every time the autodetector adds a new operation. For example, if Django detects a new model, add_operation is called with a new CreateModel operation instance.

The patch hooks into the add_operation function to transform the following operations:

• Createmodel into a PostgresCreatePartitionedModel operation if the model is a PostgresPartitionedModel and adds a PostgresAddDefaultPartition operation to create a default partition.

• DeleteModel into a PostgresDeletePartitionedModel operation if the model is a PostgresPartitionedModel.

• CreateModel into a PostgresCreateViewModel operation if the model is a PostgresViewModel.

• DeleteModel into a PostgresDeleteviewModel operation if the model is a PostgresViewModel.

• CreateModel into a PostgresCreateMaterializedViewModel operation if the model is a PostgresMaterializedViewModel.

• DeleteModel into a PostgresDeleteMaterializedViewModel operation if the model is a PostgresMaterializedViewModel.

• AddField into ApplyState migration if the model is a PostgresViewModel or PostgresMaterializedViewModel.

• AlterField into ApplyState migration if the model is a PostgresViewModel or PostgresMaterializedViewModel.

• RenameField into ApplyState migration if the model is a PostgresViewModel or PostgresMaterializedViewModel.

• RemoveField into ApplyState migration if the model is a PostgresViewModel or PostgresMaterializedViewModel.

ProjectState patch

• Patches django.db.migrations.state.ProjectState.from_apps

This function is called to build up the current migration state from all the installed apps. For each model, a ModelState is created.

The patch hooks into the from_apps function to transform the following:

• Create PostgresPartitionedModelState from the model if the model is a PostgresPartitionedModel.
• Create PostgresViewModelState from the model if the model is a PostgresViewModel.
• Create PostgresMaterializedViewModelState from the model if the model is a PostgresMaterializedViewModel.

These custom model states are needed to track partitioning and view options (PartitioningMeta and ViewMeta) in migrations. Without this, the partitioning and view optiosn would not end up in migrations.