Support modifying foreign keys when using Sqlite

[Tuckertcs]

When creating SeaORM migrations using a Sqlite database, the SchemaManager.create_foreign_key(...) and SchemaManager.drop_foreign_key(...) functions cause an error: Sqlite does not support modification of foreign key constraints to existing tables. The solution is to recreate the table with the foreign keys, but this requires multiple calls to rename the table, create a new table, copy the data over, then delete the old table.

My suggestion is to allow for the original foreign key functions to handle this under the hood, to achieve parity between Sqlite and other databases.

Here is a code example to illustrate what I'm talking about, which adds a foreign key constraint linking the existing childs table to the existing parents table (using SeaORM version 1.0.0):

use sea_orm_migration::prelude::*;
use sea_orm_migration::sea_orm::DeriveIden;

#[derive(DeriveIden)]
pub enum Parents {
	Table,
	Id,
}

#[derive(DeriveIden)]
pub enum Childs {
	Table,
	Id,
	ParentId,
}

#[derive(DeriveIden)]
#[sea_orm(iden = "old_childs_id")]
pub enum OldTooltipSlugs {
	Table,
}

#[derive(DeriveMigrationName)]
pub struct Migration;

#[async_trait::async_trait]
impl MigrationTrait for Migration {
	async fn up(&self, manager: &SchemaManager) -> Result<(), DbErr> {
		// Ideal method, but does not work with Sqlite:

		// manager
		// 	.create_foreign_key(
		// 		ForeignKey::create()
		// 			.name("fk_childs_parent_id")
		// 			.from(Childs::Table, Childs::ParentId)
		// 			.to(Parents::Table, Parents::Id)
		// 			.to_owned(),
		// 	)
		// 	.await

		// Unideal method, but works with Sqlite:

		// Rename table to old table.
		manager
			.rename_table(
				Table::rename()
					.table(Childs::Table, OldChilds::Table)
					.to_owned(),
			)
			.await?;

		// Create new table with foreign key this time.
		manager
			.create_table(
				Table::create()
					.table(Childs::Table)
					.if_not_exists()
					.col(ColumnDef::new(Childs::Id).int().not_null().primary_key())
					.col(ColumnDef::new(Childs::ParentId).string().not_null())
					.foreign_key(
						ForeignKey::create()
							.name("fk_childs_parent_id")
							.from(Childs::Table, Childs::ParentId)
							.to(Parents::Table, Parents::Id)
							.on_delete(ForeignKeyAction::Cascade),
					)
					.to_owned(),
			)
			.await?;

		// Copy data over.
		manager
			.exec_stmt(
				Query::insert()
					.into_table(Childs::Table)
					.columns([Childs::Id, Childs::ParentId])
					.values_panic([/* SELECT * FROM old_childs */])
					.to_owned(),
			)
			.await?;

		// Delete old table.
		manager
			.drop_table(Table::drop().table(OldChilds::Table).to_owned())
			.await
	}

	async fn down(&self, manager: &SchemaManager) -> Result<(), DbErr> {
		// Ideal method, but does not work with Sqlite:

		// manager
		// 	.drop_foreign_key(
		// 		ForeignKey::drop()
		// 			.name("fk_childs_parent_id")
		// 			.to_owned(),
		// 	)
		// 	.await

		// Unideal method, but works with Sqlite:

		// Rename table to old table.
		manager
			.rename_table(
				Table::rename()
					.table(Childs::Table, OldChilds::Table)
					.to_owned(),
			)
			.await?;

		// Create new table without foreign key again.
		manager
			.create_table(
				Table::create()
					.table(Childs::Table)
					.if_not_exists()
					.col(ColumnDef::new(Childs::Id).int().not_null().primary_key())
					.col(ColumnDef::new(Childs::ParentId).string().not_null())
					.to_owned(),
			)
			.await?;

		// Copy data back over.
		manager
			.exec_stmt(
				Query::insert()
					.into_table(Childs::Table)
					.columns([Childs::Id, Childs::ParentId])
					.values_panic([/* SELECT * FROM old_childs */])
					.to_owned(),
			)
			.await?;

		// Delete old table.
		manager
			.drop_table(Table::drop().table(OldChilds::Table).to_owned())
			.await
	}
}