Has many relation to be auto defined

src/entity/base_entity.rs

@@ -1,7 +1,7 @@
 use crate::{
     ActiveModelTrait, ColumnTrait, Delete, DeleteOne, FromQueryResult, Insert, ModelTrait,
-    OneOrManyActiveModel, PrimaryKeyToColumn, PrimaryKeyTrait, QueryFilter, RelationBuilder,
-    RelationTrait, RelationType, Select, Update, UpdateOne,
+    OneOrManyActiveModel, PrimaryKeyToColumn, PrimaryKeyTrait, QueryFilter, Related,
+    RelationBuilder, RelationTrait, RelationType, Select, Update, UpdateOne,
 };
 use sea_query::{Iden, IntoValueTuple};
 use std::fmt::Debug;
@@ -35,11 +35,11 @@ pub trait EntityTrait: EntityName {
         RelationBuilder::new(RelationType::HasOne, Self::default(), related)
     }
 
-    fn has_many<R>(related: R) -> RelationBuilder<Self, R>
+    fn has_many<R>(_: R) -> RelationBuilder<Self, R>
     where
-        R: EntityTrait,
+        R: EntityTrait + Related<Self>,
     {
-        RelationBuilder::new(RelationType::HasMany, Self::default(), related)
+        RelationBuilder::from_rel_def(R::to().rev())
     }
 
     /// ```

src/entity/relation.rs

@@ -78,6 +78,17 @@ where
         }
     }
 
+    pub(crate) fn from_rel_def(rel: RelationDef) -> Self {
+        Self {
+            entities: PhantomData,
+            rel_type: rel.rel_type,
+            from_tbl: rel.from_tbl,
+            to_tbl: rel.to_tbl,
+            from_col: Some(rel.from_col),
+            to_col: Some(rel.to_col),
+        }
+    }
+
     pub fn from(mut self, identifier: E::Column) -> Self {
         self.from_col = Some(identifier.into_identity());
         self

src/tests_cfg/cake.rs

@@ -52,10 +52,7 @@ impl ColumnTrait for Column {
 impl RelationTrait for Relation {
     fn def(&self) -> RelationDef {
         match self {
-            Self::Fruit => Entity::has_many(super::fruit::Entity)
-                .from(Column::Id)
-                .to(super::fruit::Column::CakeId)
-                .into(),
+            Self::Fruit => Entity::has_many(super::fruit::Entity).into(),
         }
     }
 }

src/tests_cfg/fruit.rs

@@ -36,7 +36,9 @@ impl PrimaryKeyTrait for PrimaryKey {
 }
 
 #[derive(Copy, Clone, Debug, EnumIter)]
-pub enum Relation {}
+pub enum Relation {
+    Cake,
+}
 
 impl ColumnTrait for Column {
     type EntityName = Entity;
@@ -52,7 +54,18 @@ impl ColumnTrait for Column {
 
 impl RelationTrait for Relation {
     fn def(&self) -> RelationDef {
-        panic!()
+        match self {
+            Self::Cake => Entity::has_one(super::cake::Entity)
+                .from(Column::CakeId)
+                .to(super::cake::Column::Id)
+                .into(),
+        }
+    }
+}
+
+impl Related<super::cake::Entity> for Entity {
+    fn to() -> RelationDef {
+        Relation::Cake.def()
     }
 }