feat: add reply functionality to thoughts, including database migration and tests

thoughts-backend/migration/src/m20250906_145148_add_reply_to_thoughts.rs

@@ -0,0 +1,46 @@
+use sea_orm_migration::{prelude::*, schema::*};
+
+use crate::m20250905_000001_init::Thought;
+
+#[derive(DeriveMigrationName)]
+pub struct Migration;
+
+#[async_trait::async_trait]
+impl MigrationTrait for Migration {
+    async fn up(&self, manager: &SchemaManager) -> Result<(), DbErr> {
+        manager
+            .alter_table(
+                Table::alter()
+                    .table(Thought::Table)
+                    .add_column(uuid_null(ThoughtExtension::ReplyToId))
+                    .add_foreign_key(
+                        TableForeignKey::new()
+                            .name("fk_thought_reply_to_id")
+                            .from_tbl(Thought::Table)
+                            .from_col(ThoughtExtension::ReplyToId)
+                            .to_tbl(Thought::Table)
+                            .to_col(Thought::Id)
+                            .on_delete(ForeignKeyAction::SetNull),
+                    )
+                    .to_owned(),
+            )
+            .await
+    }
+
+    async fn down(&self, manager: &SchemaManager) -> Result<(), DbErr> {
+        manager
+            .alter_table(
+                Table::alter()
+                    .table(Thought::Table)
+                    .drop_foreign_key(Alias::new("fk_thought_reply_to_id"))
+                    .drop_column(ThoughtExtension::ReplyToId)
+                    .to_owned(),
+            )
+            .await
+    }
+}
+
+#[derive(DeriveIden)]
+enum ThoughtExtension {
+    ReplyToId,
+}