Refactor GenerateUuid4 function

.gitignore

@@ -2,6 +2,7 @@
 *.egg-info
 *.bak
 *.db
+.env
 .coverage
 .tox
 .venv

anonymiser/db/__init__.py

@@ -1,3 +1,3 @@
-from .expressions import GenerateUuid4
+from .functions import GenerateUuid4
 
 __all__ = ["GenerateUuid4"]

anonymiser/db/functions.py

@@ -0,0 +1,119 @@
+from typing import Any
+
+from django.db import models
+from django.db.backends.base.base import BaseDatabaseWrapper
+from django.db.models.sql.compiler import SQLCompiler
+
+
+class GenerateUuid4(models.Func):
+    """
+    Generate a new UUID (v4) value.
+
+    Most databases support some form of UUID generation, but the syntax
+    varies between them. This expression is only implemented for SQLite
+    and PostgreSQL.
+
+    The expression can be used to generate a UUID value for a field
+    where the value needs to be generated by the database, rather than
+    by the application.
+
+    As an example, if you have a model with a UUID field and you want to
+    roll all the ids, if you try to do this:
+
+        >>> User.objects.all().update(uuid=uuid.uuid4())
+
+    You will end up with every row in the database having the same UUID.
+    This is because the uuid.uuid4() function is evaluated once and the
+    same value is used for every row:
+
+        UPDATE "user" SET "uuid" =
+        "3742138f-1399-47b2-a721-1710abefded6"
+
+    If you want to generate a new UUID for each row whilst updating with
+    a single "UPDATE" SQL statement, you can use this expression:
+
+        >>> User.objects.all().update(uuid=GenerateUuid4())
+
+    Which will generate something like this:
+
+        UPDATE "user" SET "uuid" = <function_to_generate_uuid>
+
+    The specific function that is called is vendor specific.
+
+    """
+
+    output_field = models.UUIDField()
+
+    def as_sql(
+        self,
+        compiler: SQLCompiler,
+        connection: BaseDatabaseWrapper,
+        **extra_context: Any,
+    ) -> tuple[str, list]:
+        """
+        Generate the SQL fragment for the expression.
+
+        Note that this is only called when `as_{{vendor}}` method does
+        not exist for the expression.
+
+        (From https://docs.djangoproject.com/en/4.2/ref/models/lookups/)
+
+        "Returns a tuple (sql, params), where sql is the SQL string, and
+        params is the list or tuple of query parameters. The compiler is
+        an SQLCompiler object, which has a compile() method that can be
+        used to compile other expressions. The connection is the
+        connection used to execute the query.
+
+        Calling expression.as_sql() is usually incorrect - instead
+        compiler.compile(expression) should be used. The
+        compiler.compile() method will take care of calling
+        vendor-specific methods of the expression.
+
+        Custom keyword arguments may be defined on this method if it's
+        likely that as_vendorname() methods or subclasses will need to
+        supply data to override the generation of the SQL string. See
+        Func.as_sql() for example usage."
+
+        """
+        raise NotImplementedError(
+            f"GenerateUuid4 is not implemented for {connection.vendor}"
+        )
+
+    def as_sqlite(
+        self,
+        compiler: SQLCompiler,
+        connection: BaseDatabaseWrapper,
+        **extra_context: Any,
+    ) -> tuple[str, list]:
+        """
+        Generate the SQL fragment for the expression in SQLite format.
+
+        (From https://docs.djangoproject.com/en/4.2/ref/models/lookups/)
+
+        "Works like as_sql() method. When an expression is compiled by
+        compiler.compile(), Django will first try to call
+        as_vendorname(), where vendorname is the vendor name of the
+        backend used for executing the query. The vendorname is one of
+        postgresql, oracle, sqlite, or mysql for Django's built-in
+        backends."
+
+        """
+        return super().as_sql(
+            compiler,
+            connection,
+            function="HEX(RANDOMBLOB(16))",
+            # override the default template as otherwise we end up
+            # trying to append parentheses: HEX(RANDOMBLOB(16))()
+            template="%(function)s",
+            **extra_context,
+        )
+
+    def as_postgresql(
+        self,
+        compiler: SQLCompiler,
+        connection: BaseDatabaseWrapper,
+        **extra_context: Any,
+    ) -> tuple[str, list]:
+        return super().as_sql(
+            compiler, connection, function="gen_random_uuid", **extra_context
+        )

tests/test_expressions.py

@@ -4,7 +4,7 @@
 from django.db import connection
 from django.db.backends.utils import CursorWrapper
 
-from anonymiser.db.expressions import GenerateUuid4
+from anonymiser.db.functions import GenerateUuid4
 
 from .models import User
 
@@ -14,7 +14,7 @@
     "vendor,sql_func",
     [
         ("sqlite", "HEX(RANDOMBLOB(16))"),
-        ("postgresql", "get_random_uuid()"),
+        ("postgresql", "gen_random_uuid()"),
     ],
 )
 @mock.patch.object(CursorWrapper, "execute")

tests/test_models.py

@@ -3,7 +3,7 @@
 import pytest
 from django.db import models
 
-from anonymiser.db.expressions import GenerateUuid4
+from anonymiser.db.functions import GenerateUuid4
 from anonymiser.models import FieldSummaryData
 
 from .anonymisers import BadUserAnonymiser, UserAnonymiser, UserRedacter