Use different export job fields for the different storage backends

This way, the database definition is not depdendent on the runtime configuration.

bookwyrm/migrations/0198_export_job_separate_file_fields.py

@@ -0,0 +1,28 @@
+# Generated by Django 3.2.25 on 2024-03-24 11:20
+
+import bookwyrm.storage_backends
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("bookwyrm", "0197_merge_20240324_0235"),
+    ]
+
+    operations = [
+        migrations.RenameField(
+            model_name="bookwyrmexportjob",
+            old_name="export_data",
+            new_name="export_data_file",
+        ),
+        migrations.AddField(
+            model_name="bookwyrmexportjob",
+            name="export_data_s3",
+            field=models.FileField(
+                null=True,
+                storage=bookwyrm.storage_backends.ExportsS3Storage,
+                upload_to="",
+            ),
+        ),
+    ]

bookwyrm/models/bookwyrm_export_job.py

@@ -37,15 +37,30 @@ class BookwyrmAwsSession(BotoSession):
 class BookwyrmExportJob(ParentJob):
     """entry for a specific request to export a bookwyrm user"""
 
-    if settings.USE_S3:
-        storage = storage_backends.ExportsS3Storage
-    else:
-        storage = storage_backends.ExportsFileStorage
+    # Only one of these fields is used, dependent on the configuration.
+    export_data_file = FileField(null=True, storage=storage_backends.ExportsFileStorage)
+    export_data_s3 = FileField(null=True, storage=storage_backends.ExportsS3Storage)
 
-    export_data = FileField(null=True, storage=storage)
     export_json = JSONField(null=True, encoder=DjangoJSONEncoder)
     json_completed = BooleanField(default=False)
 
+    @property
+    def export_data(self):
+        """returns the file field of the configured storage backend"""
+        # TODO: We could check whether a field for a different backend is
+        # filled, to support migrating to a different backend.
+        if settings.USE_S3:
+            return self.export_data_s3
+        return self.export_data_file
+
+    @export_data.setter
+    def export_data(self, value):
+        """sets the file field of the configured storage backend"""
+        if settings.USE_S3:
+            self.export_data_s3 = value
+        else:
+            self.export_data_file = value
+
     def start_job(self):
         """Start the job"""
 
@@ -284,7 +299,7 @@ def start_export_task(**kwargs):
         # prepare the initial file and base json
         job.export_data = ContentFile(b"", str(uuid4()))
         job.export_json = job.user.to_activity()
-        job.save(update_fields=["export_data", "export_json"])
+        job.save(update_fields=["export_data_file", "export_data_s3", "export_json"])
 
         # let's go
         json_export.delay(job_id=job.id, job_user=job.user.id, no_children=False)