Self referencing belongs_to relations

[billy1624]

Motivation

One could define child-to-parent relation and query parent::Model on seaography. However, the parent-to-child relation will fetch child::Model but not Vec<child::Model>.

#[derive(Copy, Clone, Debug, EnumIter, DeriveRelation, RelationsCompact)]
pub enum Relation {
    #[sea_orm(
        belongs_to = "super::deck::Entity",
        from = "super::deck::Column::ParentId",
        to = "super::deck::Column::Id",
    )]
    Parent,
    #[sea_orm(
        belongs_to = "super::deck::Entity",
        from = "super::deck::Column::Id",
        to = "super::deck::Column::ParentId",
    )]
    Child,
}

Proposed Solutions

A #[seaography(has_many)] proc attribute to override the default: fetching Model instead of Vec<Model> for belongs_to relation.

#[derive(Copy, Clone, Debug, EnumIter, DeriveRelation, RelationsCompact)]
pub enum Relation {
    #[sea_orm(
        belongs_to = "super::deck::Entity",
        from = "super::deck::Column::Id",
        to = "super::deck::Column::ParentId",
    )]
    #[seaography(has_many)]
    Child,
}

I believe we need to change this part in the derive macro.

seaography/derive/src/relation.rs

Lines 188 to 200 in 04c3034

	let (return_type, extra_imports, map_method) = if has_many.is_some() {
	(
	quote! { Vec<#path::Model> },
	quote! { use seaography::itertools::Itertools; },
	quote! { .into_group_map() },
	)
	} else if belongs_to.is_some() {
	(quote! { #path::Model }, quote! {}, quote! { .collect() })
	} else {
	return Err(crate::error::Error::Internal(
	"Cannot map relation: neither one-many or many-one".into(),
	));
	};

Additional Information

Context: https://discord.com/channels/873880840487206962/1039063562083127326

[karatakis]

I can work on this after #84 is merged