Relations between entities

Many to One relation

Let's consider the following use case, we have a todo app, with a Task entity. We want to assign each Task to a category.

We'll add the Category entity. Category.ts

import { Entity, Field, IdEntity } from "remult";
import { Task } from "./Task";

@Entity('categories', {
    allowApiCrud: true
})
export class Category extends IdEntity {
    @Fields.string()
    name = '';
}

and we'll use it as a field in the Task entity.

Note that we set the valueType option of the Field decorator, because create-react-app doesn't support emitDecoratorMetadata

import { Entity, Field, IdEntity } from "remult";
import { Category } from "./Category";

@Entity('tasks', {
    allowApiCrud: true
})
export class Task extends IdEntity {
    @Fields.string()
    title = '';
    @Fields.boolean()
    completed = false;
    @Field(() => Category)
    category?: Category;
}

When a Task is loaded, its Category is also automatically loaded, using a cache mechanism to prevent the reload of an already loaded Category. To disable that, set the lazy option to true. For more information see lazy loading of related entities

One to Many Relationship

Now consider that we want to keep a history of changes that happen to our Task object. We will create a HistoryRecord entity that will store records of changes to our task

import { Entity, IdEntity } from "remult";

@Entity('historyRecords', {
    allowApiCrud: true
})
export class HistoryRecord extends IdEntity {
    @Fields.string()
    taskId: String;

    @Fields.string()
    event: String;
}

Each HistoryRecord object will reference its corresponding Task by taskId.

Now, we can update Task to include an array of HistoryRecord objects when it is loaded:

import { Entity, Field, IdEntity } from "remult";
import { Category } from "./Category";
import { HistoryRecord } from "./HistoryRecord";

@Entity('tasks', {
    allowApiCrud: true
})
export class Task extends IdEntity {
    @Fields.string()
    title = '';
    @Fields.boolean()
    completed = false;
    @Field(() => Category)
    category?: Category;
    @Fields.object<Task>((options, remult) => {
        options.serverExpression = async (task) =>
            remult.repo(HistoryRecord).find({ where: { taskId: task.id } })
    })
    history: HistoryRecord[]

}

options.serverExpression is a function that can resolve a field from anywhere. In this example, we're simply calling the HistoryRecord table to resolve all records where the taskId matches the id of the task being resolved. Despite being decorated with @Fields.object..., the history attribute will not be stored on the tasks table in the database.

Relations between entities ​

Many to One relation ​

One to Many Relationship ​

Relations between entities

Many to One relation

One to Many Relationship