fix: allow 'null' values for optional fields in custom types

packages/schema/src/plugins/enhancer/enhance/model-typedef-generator.ts

@@ -17,8 +17,11 @@ export function generateTypeDefType(sourceFile: SourceFile, decl: TypeDef) {
                         field.comments.forEach((c) => writer.writeLine(`     * ${unwrapTripleSlashComment(c)}`));
                         writer.writeLine(`     */`);
                     }
+                    // optional fields are also nullable (to be consistent with Prisma)
                     writer.writeLine(
-                        `    ${field.name}${field.type.optional ? '?' : ''}: ${zmodelTypeToTsType(field.type)};`
+                        `    ${field.name}${field.type.optional ? '?' : ''}: ${zmodelTypeToTsType(field.type)}${
+                            field.type.optional ? ' | null' : ''
+                        };`
                     );
                 });
             });

tests/regression/tests/issue-1857.test.ts

@@ -0,0 +1,45 @@
+import { loadSchema } from '@zenstackhq/testtools';
+
+describe('issue 1857', () => {
+    it('regression', async () => {
+        const { zodSchemas } = await loadSchema(
+            `
+            type JSONContent {
+                type String
+                text String?
+            }
+
+            model Post {
+                id String @id @default(uuid())
+                content JSONContent @json
+                @@allow('all', true)
+            }
+            `,
+            {
+                provider: 'postgresql',
+                pushDb: false,
+                compile: true,
+                extraSourceFiles: [
+                    {
+                        name: 'main.ts',
+                        content: `
+            import { PrismaClient } from '@prisma/client';
+            import { enhance } from '.zenstack/enhance';
+
+            async function main() {
+                const prisma = new PrismaClient();
+                await prisma.post.create({
+                    data: {
+                        content: { type: 'foo', text: null }
+                    }
+                });
+            }
+                `,
+                    },
+                ],
+            }
+        );
+
+        zodSchemas.models.JSONContentSchema.parse({ type: 'foo', text: null });
+    });
+});