ENG-3298: Add parent_id column to Comment model for threading

.fides/db_dataset.yml

@@ -378,6 +378,9 @@ dataset:
     - name: id
       data_categories:
       - system.operations
+    - name: parent_id
+      data_categories:
+      - system.operations
     - name: updated_at
       data_categories:
       - system.operations

changelog/7864-comment-threading-migration.yaml

@@ -0,0 +1,4 @@
+type: Added
+description: Added parent_id column to Comment model for threading support
+pr: 7864
+labels: ["db-migration"]

src/fides/api/alembic/migrations/versions/xx_2026_04_08_1901_1724da7ee981_add_comment_parent_id.py

@@ -0,0 +1,36 @@
+"""add_comment_parent_id
+
+Revision ID: 1724da7ee981
+Revises: 6a42f48c23dd
+Create Date: 2026-04-08 19:01:40.083985
+
+"""
+import sqlalchemy as sa
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision = "1724da7ee981"
+down_revision = "6a42f48c23dd"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.add_column(
+        "comment", sa.Column("parent_id", sa.String(length=255), nullable=True)
+    )
+    op.create_index("ix_comment_parent_id", "comment", ["parent_id"], unique=False)
+    op.create_foreign_key(
+        "comment_parent_id_fkey",
+        "comment",
+        "comment",
+        ["parent_id"],
+        ["id"],
+        ondelete="SET NULL",
+    )
+
+
+def downgrade():
+    op.drop_constraint("comment_parent_id_fkey", "comment", type_="foreignkey")
+    op.drop_index("ix_comment_parent_id", table_name="comment")
+    op.drop_column("comment", "parent_id")

src/fides/api/models/comment.py

@@ -96,12 +96,35 @@ class Comment(Base):
     user_id = Column(
         String, ForeignKey("fidesuser.id", ondelete="SET NULL"), nullable=True
     )
+    parent_id = Column(
+        String(255),
+        ForeignKey("comment.id", name="comment_parent_id_fkey", ondelete="SET NULL"),
+        nullable=True,
+    )
     # Not all users in the system have a username, and users can be deleted.
     # Store a non-normalized copy of username for these cases.
     username = Column(String, nullable=True)
     comment_text = Column(String, nullable=False)
     comment_type = Column(EnumColumn(CommentType), nullable=False)
 
+    __table_args__ = (Index("ix_comment_parent_id", "parent_id"),)
+
+    parent = relationship(
+        "Comment",
+        remote_side="Comment.id",
+        foreign_keys=[parent_id],
+        back_populates="replies",
+        uselist=False,
+    )
+
+    replies = relationship(
+        "Comment",
+        foreign_keys=[parent_id],
+        back_populates="parent",
+        uselist=True,
+        order_by="Comment.created_at",
+    )
+
     user = relationship(
         "FidesUser",
         lazy="selectin",
@@ -129,8 +152,12 @@ class Comment(Base):
     )
 
     def delete(self, db: Session) -> None:
-        """Delete the comment and all associated references."""
-        # Delete attachments associated with this comment
+        """Delete the comment, its replies, and all associated references."""
+        # TODO (ENG-3299): When message_to_subject / reply_from_subject CommentTypes
+        # are added, prevent deletion of comments with those types to preserve
+        # correspondence threads.
+        for reply in self.replies:
+            reply.delete(db)
         AttachmentService(db).delete_for_reference(
             self.id, AttachmentReferenceType.comment
         )

tests/ctl/models/test_comment.py

@@ -6,7 +6,12 @@
     AttachmentReference,
     AttachmentReferenceType,
 )
-from fides.api.models.comment import Comment, CommentReference, CommentReferenceType
+from fides.api.models.comment import (
+    Comment,
+    CommentReference,
+    CommentReferenceType,
+    CommentType,
+)
 from fides.api.models.fides_user import FidesUser
 
 
@@ -395,6 +400,55 @@ def mock_get_s3_client(auth_method, storage_secrets):
     attachment.delete(db)
 
 
+def test_reply_relationship(db, comment):
+    """Test that parent/replies relationships load correctly on a threaded comment."""
+    reply = Comment.create(
+        db,
+        data={
+            "user_id": comment.user_id,
+            "comment_text": "This is a reply",
+            "comment_type": CommentType.reply,
+            "parent_id": comment.id,
+        },
+    )
+    db.refresh(comment)
+
+    assert reply.parent_id == comment.id
+    assert reply.parent.id == comment.id
+    assert reply in comment.replies
+
+    reply.delete(db)
+
+
+def test_delete_parent_comment_deletes_replies(db, user):
+    """Test that deleting a parent comment also deletes its replies via delete()."""
+    parent = Comment.create(
+        db,
+        data={
+            "user_id": user.id,
+            "comment_text": "Parent comment",
+            "comment_type": CommentType.note,
+        },
+    )
+    reply = Comment.create(
+        db,
+        data={
+            "user_id": user.id,
+            "comment_text": "Reply comment",
+            "comment_type": CommentType.reply,
+            "parent_id": parent.id,
+        },
+    )
+    parent_id = parent.id
+    reply_id = reply.id
+
+    parent.delete(db)
+    db.commit()
+
+    assert db.query(Comment).filter_by(id=parent_id).first() is None
+    assert db.query(Comment).filter_by(id=reply_id).first() is None
+
+
 def test_comment_to_multiple_attachments_relationship(
     s3_client, db, comment, multiple_attachments, monkeypatch
 ):