fix: implicit many to many relation generation (#71)

* fix: implicit many to many relation generation

* test: add test for filtering many to one relations

* style: fix lint

* Add changeset

---------

Co-authored-by: Valtýr Örn Kjartansson <valtyr@gmail.com>

Author: dextertanyj

.changeset/thin-countries-work.md

@@ -0,0 +1,5 @@
+---
+"prisma-kysely": minor
+---
+
+Implicit many to many relations are finally fixed thanks to @dextertanyj 🇸🇬🎉🥂. Huge thanks to him!

src/helpers/generateImplicitManyToManyModels.test.ts

@@ -113,3 +113,180 @@ test("it respects overrides when generating field types", () => {
     },
   ]);
 });
+
+test("it filters out many-to-one relations safely", () => {
+  const newModels = generateImplicitManyToManyModels([
+    {
+      name: "Category",
+      fields: [
+        {
+          name: "id",
+          type: "String",
+          isId: true,
+          hasDefaultValue: true,
+          isList: false,
+          isReadOnly: false,
+          isRequired: true,
+          isUnique: true,
+          kind: "scalar",
+        },
+        {
+          name: "posts",
+          kind: "object",
+          isList: true,
+          isRequired: true,
+          isUnique: false,
+          isId: false,
+          isReadOnly: false,
+          type: "Post",
+          hasDefaultValue: false,
+          relationName: "CategoryToPost",
+          relationFromFields: [],
+          relationToFields: [],
+          isGenerated: false,
+          isUpdatedAt: false,
+        },
+      ],
+      primaryKey: null,
+      dbName: null,
+      uniqueFields: [],
+      uniqueIndexes: [],
+    },
+    {
+      name: "Post",
+      fields: [
+        {
+          name: "id",
+          type: "String",
+          isId: true,
+          hasDefaultValue: true,
+          isList: false,
+          isReadOnly: false,
+          isRequired: true,
+          isUnique: true,
+          kind: "scalar",
+        },
+        {
+          name: "categories",
+          kind: "object",
+          isList: true,
+          isRequired: true,
+          isUnique: false,
+          isId: false,
+          isReadOnly: false,
+          type: "Category",
+          hasDefaultValue: false,
+          relationName: "CategoryToPost",
+          relationFromFields: [],
+          relationToFields: [],
+          isGenerated: false,
+          isUpdatedAt: false,
+        },
+        {
+          name: "userId",
+          kind: "scalar",
+          isList: false,
+          isRequired: true,
+          isUnique: false,
+          isId: false,
+          isReadOnly: true,
+          hasDefaultValue: false,
+          type: "String",
+          isGenerated: false,
+          isUpdatedAt: false,
+        },
+        {
+          name: "user",
+          kind: "object",
+          isList: false,
+          isRequired: true,
+          isUnique: false,
+          isId: false,
+          isReadOnly: false,
+          hasDefaultValue: false,
+          type: "User",
+          relationName: "PostToUser",
+          relationFromFields: ["userId"],
+          relationToFields: ["id"],
+          relationOnDelete: "Cascade",
+          isGenerated: false,
+          isUpdatedAt: false,
+        },
+      ],
+      primaryKey: null,
+      dbName: null,
+      uniqueFields: [],
+      uniqueIndexes: [],
+    },
+    {
+      name: "User",
+      fields: [
+        {
+          name: "id",
+          type: "String",
+          isId: true,
+          hasDefaultValue: true,
+          isList: false,
+          isReadOnly: false,
+          isRequired: true,
+          isUnique: true,
+          kind: "scalar",
+        },
+        {
+          name: "posts",
+          kind: "object",
+          isList: true,
+          isRequired: true,
+          isUnique: false,
+          isId: false,
+          isReadOnly: false,
+          type: "Post",
+          hasDefaultValue: false,
+          relationName: "PostToUser",
+          relationFromFields: [],
+          relationToFields: [],
+          isGenerated: false,
+          isUpdatedAt: false,
+        },
+      ],
+      primaryKey: null,
+      dbName: null,
+      uniqueFields: [],
+      uniqueIndexes: [],
+    },
+  ]);
+
+  expect(newModels).toEqual<DMMF.Model[]>([
+    {
+      name: "CategoryToPost",
+      dbName: "_CategoryToPost",
+      fields: [
+        {
+          hasDefaultValue: false,
+          isId: false,
+          isList: false,
+          isReadOnly: true,
+          isRequired: true,
+          isUnique: false,
+          kind: "scalar",
+          name: "A",
+          type: "String",
+        },
+        {
+          hasDefaultValue: false,
+          isId: false,
+          isList: false,
+          isReadOnly: true,
+          isRequired: true,
+          isUnique: false,
+          kind: "scalar",
+          name: "B",
+          type: "String",
+        },
+      ],
+      primaryKey: null,
+      uniqueFields: [],
+      uniqueIndexes: [],
+    },
+  ]);
+});

src/helpers/generateImplicitManyToManyModels.ts

@@ -111,18 +111,21 @@ function getJoinIdType(joinField: DMMF.Field, models: DMMF.Model[]): string {
   return idField.type;
 }
 
-function filterManyToManyRelationFields(models: DMMF.Model[]): DMMF.Field[] {
-  return models
-    .map((model) =>
-      model.fields
-        .filter(
-          (field) =>
-            field.relationName &&
-            field.isList &&
-            field.relationFromFields?.length === 0 &&
-            field.relationToFields?.length === 0
-        )
-        .map((field) => field)
-    )
-    .flat();
+function filterManyToManyRelationFields(models: DMMF.Model[]) {
+  const fields = models.flatMap((model) => model.fields);
+
+  const relationFields = fields.filter(
+    (field): field is DMMF.Field & Required<Pick<DMMF.Field, "relationName">> =>
+      !!field.relationName
+  );
+
+  const nonManyToManyRelationNames = relationFields
+    .filter((field) => !field.isList)
+    .map((field) => field.relationName);
+
+  const notManyToMany = new Set<string>(nonManyToManyRelationNames);
+
+  return relationFields.filter(
+    (field) => !notManyToMany.has(field.relationName)
+  );
 }