fix: Support implicit many-to-many (#286)

Author: jiashengguo

packages/schema/src/language-server/validator/datamodel-validator.ts

@@ -256,15 +256,18 @@ export default class DataModelValidator implements AstValidator<DataModel> {
                 relationOwner = field;
             }
         } else {
-            [field, oppositeField].forEach((f) => {
-                if (!this.isSelfRelation(f, thisRelation.name)) {
-                    accept(
-                        'error',
-                        'Field for one side of relation must carry @relation attribute with both "fields" and "references" fields',
-                        { node: f }
-                    );
-                }
-            });
+            // if both the field is array, then it's an implicit many-to-many relation
+            if (!(field.type.array && oppositeField.type.array)) {
+                [field, oppositeField].forEach((f) => {
+                    if (!this.isSelfRelation(f, thisRelation.name)) {
+                        accept(
+                            'error',
+                            'Field for one side of relation must carry @relation attribute with both "fields" and "references" fields',
+                            { node: f }
+                        );
+                    }
+                });
+            }
             return;
         }
 

packages/schema/tests/schema/validation/datamodel-validation.test.ts

@@ -257,6 +257,23 @@ describe('Data Model Validation Tests', () => {
             }
         `);
 
+        // many-to-many implicit
+        //https://www.prisma.io/docs/concepts/components/prisma-schema/relations/many-to-many-relations#implicit-many-to-many-relations
+        await loadModel(`
+        ${prelude}
+        model Post {
+            id         Int        @id @default(autoincrement())
+            title      String
+            categories Category[]
+          }
+          
+          model Category {
+            id    Int    @id @default(autoincrement())
+            name  String
+            posts Post[]
+          }
+        `);
+
         // one-to-one incomplete
         expect(
             await loadModelWithError(`