Merge pull request #22 from hpi-schul-cloud/microORM-entity-experience

Defining entity in mikro-orm

docs/schulcloud-server/Coding-Guidelines/micro-orm.md

@@ -0,0 +1,61 @@
+# Defining Entities
+
+When defining entities with MikroORM (Version 5), the following should be considered:
+The property decorator requires explicit assignment of the type to the property and may not work correctly when working with type inference or assigning union types to a property. In these cases, the metadata may not be set correctly, which can lead to exceptions, for example, when using the `em.assign()` or `em.aggregate()` functions.
+
+Therefore, the following is **not** sufficient:
+
+```TypeScript
+  @Property()
+  termsAccepted = false;
+
+ @Property()
+ createdAt = new Date();
+
+```
+
+The following works:
+
+```TypeScript
+ @Property()
+ termsAccepted: boolean = false;
+
+ @Property()
+ createdAt: Date = new Date();
+
+```
+
+The better way is to provide the type through the decorator:
+
+```TypeScript
+ @Property({ type: 'boolean' })
+ termsAccepted = false;
+
+ @Property({ type: Date })
+ createdAt = new Date();
+
+```
+
+Errors can also occur when specifying multiple types (union types):
+
+```TypeScript
+ @Poperty({ nullable: true })
+ dueDate: Date | null;
+
+```
+
+To set the metadata correctly, do the following:
+
+```TypeScript
+ @Property({ type: Date, nullable: true })
+ dueDate: Date | null;
+
+```
+
+If type inference is not used, specifying the type through the property decorator is not necessary:
+
+```TypeScript
+ @Property()
+ name: string;
+
+```