Backfill LMSUserAssignmentMembership based on AssignmentMembership rows

Now that the python code creates LMSUserAssignmentMembership rows on
each assignment backfill the existing data in one migration.

lms/migrations/versions/a9e5a33e96b7_backfill_lmsuserassignmentmembership.py

@@ -0,0 +1,50 @@
+"""Backfill LMSUserAssignmentMembership."""
+
+import sqlalchemy as sa
+from alembic import op
+
+revision = "a9e5a33e96b7"
+down_revision = "712c4c9a4e2e"
+
+
+def upgrade() -> None:
+    conn = op.get_bind()
+    conn.execute(
+        sa.text(
+            """
+        WITH backfill as (
+            SELECT
+                assignment_membership.created,
+                assignment_membership.updated,
+                assignment_membership.assignment_id,
+                lms_user.id as lms_user_id,
+                lti_role_id
+            FROM assignment_membership
+            JOIN "user" on assignment_membership.user_id = "user".id
+            JOIN "lms_user" on "user".h_userid = "lms_user".h_userid
+        )
+        INSERT INTO lms_user_assignment_membership (
+             created,
+             updated,
+             assignment_id,
+             lms_user_id,
+             lti_role_id
+        )
+        SELECT
+           created,
+           updated,
+           assignment_id,
+           lms_user_id,
+           lti_role_id
+        FROM backfill
+        -- We are already inserting rows in the table on the python code, leave those alone
+        ON CONFLICT ON CONSTRAINT uq__lms_user_assignment_membership__assignment_id DO NOTHING
+    """
+        )
+    )
+
+    pass
+
+
+def downgrade() -> None:
+    pass