Relationships
Relationships
Introduction
Using Parse, you can store data objects establishing relations between them. To model this behavior, any ParseObject can be used as a value in other ParseObject. Internally, the Parse framework will store the referred-to object in just one place, to maintain consistency. That can give you extra power when building and running complex queries. There are three main relation types:
- one-to-many, where one object can be related to many other objects;
- many-to-many, which can create many complex relations between many objects.
- one-to-one, establishing direct relations between two objects and only them;
There are two ways to create a one-to-many relation in Parse:
- (Recomended) The first is using the Parse Pointers in Child Class, which is the fastest in creation and query time. We will use this in this guide.
- The second is using Arrays of Parse Pointers in Parent Class which can lead to slow query times depending on their size. Because of this performance issue, we will use only pointers examples.
There are three ways to create a many-to-many relation in Parse.
- (Recomended) The first is using Parse Relations, which is the fastest in creation and query time. We will use this in this guide.
- The second is using Arrays of Parse Pointers which can lead to slow query times depending on their size.
- The third is using JoinTable where the idea from classical database. When there is a many-to-nany relation, we combine every objectId or Pointer from both sides together to build a new separate table in which the relationship is tracked.
In this guide, you will implement a React book registration application that contains the three main kinds of data associations. You will learn how to create and query data relations using Back4App and React.
At any time, you can access this project via our GitHub repositories to checkout the styles and complete code.
Prerequisites
To complete this tutorial, you will need:
Goal
To perform and demonstrate database relations in React using Parse in a realistic scenario.
1 - Understanding the Book class
Since in this guide we will be using a book registration application example, you need to first understand how the object relations are laid out in this database. The main object class that you’ll be using is the Book class, which will store each book entry in the registration. These are the other four object classes:
- Publisher: book publisher name, one-to-many relation with Book;
- Genre: book genre, one-to-many relation with Book. Note that for this example we will consider that a book can only have one genre;
- Author: book author, many-to-many relation with Book, since a book can have more than one author and an author can have more than one book as well;
- ISDB: book ISDB identifying number, one-to-one relation with Book, since this number is unique for each book.
Here is a visual representation of these database tables:
For simplicity, we will assume that each object class has only a string type name attribute (title for the Book), apart from any additional relational attribute.
Step 2 - Creating relations
Before going into this step we recommend you to clone and run the React app example (JavaScript Example Repository, TypeScript Example Repository). This application has two main screens: one responsible for listing the registered books and the other for creating new books. In the book registration form, there are direct links to the other related objects and a text input field assigned to the book’s ISBD value, which will be used to create the one-to-one relation.
Let’s take a look at the book creation method that is called when submitting this form:
1 const createBook = async function (): Promise<boolean> {
2 try {
3 // These values come from state variables linked to
4 // the screen form fields, retrieving the user choices
5 // as a complete Parse.Object, when applicable;
6 const bookTitleValue: string = bookTitle;
7 const bookISBDValue: string = bookISBD;
8 // For example, bookPublisher holds the value from
9 // RadioButton.Group field with its options being every
10 // Publisher parse object instance saved on server, which is
11 // queried on screen load via useEffect
12 const bookPublisherObject: Parse.Object = bookPublisher;
13 const bookGenreObject: Parse.Object = bookGenre;
14 // bookAuthors can be an array of Parse.Objects, since the book
15 // may have more than one Author
16 const bookAuthorsObjects: [Parse.Object] = bookAuthors;
17
18 // Creates a new parse object instance
19 let Book: Parse.Object = new Parse.Object('Book');
20
21 // Set data to parse object
22 // Simple title field
23 Book.set('title', bookTitleValue);
24
25 // ONE-TO-ONE (1:1)
26 // 1:1 relation, need to check for uniqueness of value before creating a new ISBD object
27 let isbdQuery: Parse.Query = new Parse.Query('ISBD');
28 isbdQuery.equalTo('name', bookISBDValue);
29 let isbdQueryResult: Parse.Object = await isbdQuery.first();
30 if (isbdQueryResult !== null && isbdQueryResult !== undefined) {
31 // If first returns a valid object instance, it means that there
32 // is at least one instance of ISBD with the informed value
33 alert(
34 'Error! There is already an ISBD instance with this value!',
35 );
36 return false;
37 } else {
38 // Create a new ISBD object instance to create a one-to-one relation on saving
39 let ISBD: Parse.Object = new Parse.Object('ISBD');
40 ISBD.set('name', bookISBDValue);
41 ISBD = await ISBD.save();
42 // Set the new object to the new book object ISBD field
43 Book.set('isbd', ISBD);
44 }
45
46 // ONE-TO-MANY (1:N)
47 // One-to-many relations can be set in two ways:
48 // add direct object to field (Parse will convert to pointer on save)
49 Book.set('publisher', bookPublisherObject);
50 // or add pointer to field
51 Book.set('genre', bookGenreObject.toPointer());
52
53 // MANY-TO-MANY (N:N)
54 // Create a new relation so data can be added
55 let authorsRelation = Book.relation('authors');
56 // bookAuthorsObjects is an array of Parse.Objects,
57 // you can add to relation by adding the whole array or object by object
58 authorsRelation.add(bookAuthorsObjects);
59
60 // After setting the values, save it on the server
61 try {
62 await Book.save();
63 // Success
64 alert('Success!');
65 // Navigate back to home screen using react-router
66 history.push('/');
67 return true;
68 } catch (error) {
69 // Error can be caused by lack of Internet connection
70 alert(`Error! ${error.message}`);
71 return false;
72 }
73 } catch (error) {
74 // Error can be caused by lack of value selection
75 alert(
76 'Error! Make sure to select valid choices in Publisher, Genre and Author fields!',
77 );
78 return false;
79 }
80 };Let’s now look separately at how the three types of associations are done when creating the Book object.
One-to-many relation
Note how the bookPublisherObject and bookGenreObject are set to the new book Parse.Object instance. See how simple it is in Parse to create a one-to-many relation: you could either assign the target object instance or a pointer to it using the Parse.Object.set method, which takes two arguments: the field name and the value to be set. Parse will create a pointer data type column and a direct link on your dashboard for quick access under the hood.
Many-to-many relation
Look at how the bookAuthorsObjects are set to the new book Parse.Object instance. To create a many-to-many relation, you first need to create a new Parse.Object.relation and then add the related objects to it either one by one or by passing an array of Parse.Object using the Parse.Object.relation.add method. Parse will create a relation type column and also a relational table on your database. Parse will also create a link for easy access to this new table in the field column in the dashboard.
One-to-one relation
See how the bookISBDValue is set to the new book Parse.Object instance. Creating and saving one-to-one and one-to-many relations in Parse are similar processes, in which you pass as an argument the Parse.Object instance using the Parse.Object.set method, which takes two arguments: the field name and the value to be set.
The catch here is that, before saving, you need to enforce that there are no ISBD objects containing the informed ISBD ID string value in your database and that there are no Book objects already related to it as well. The second part will always be true in this case, since you are creating a new Book object every time. Enforcing the ISBD uniqueness can be achieved by using the following highlighted query:
1 let isbdQuery: Parse.Query = new Parse.Query('ISBD');
2 isbdQuery.equalTo('name', bookISBDValue);
3 let isbdQueryResult: Parse.Object = await isbdQuery.first();
4 if (isbdQueryResult !== null && isbdQueryResult !== undefined) {
5 // If first returns a valid object instance, it means that there
6 // is at least one instance of ISBD with the informed value
7 alert(
8 'Error! There is already an ISBD instance with this value!',
9 );
10 return false;
11 }After successfully saving your objects, Parse will create a pointer data type column and a direct link on your dashboard as well.
3 - Querying relations
Querying related objects is pretty straightforward as much of it is handled by Parse. Take a look at the query function in the book registers list screen and after we will highlight how each relation type is queried:
1 const queryBooks = async function (): Promise<boolean> {
2 // These values come from state variables linked to
3 // the screen query RadioButton.Group fields, with its options being every
4 // parse object instance saved on server from the referred class, which is
5 // queried on screen load via useEffect; these variables retrievie the user choices
6 // as a complete Parse.Object;
7 const queryPublisherValue: Parse.Object = queryPublisher;
8 const queryGenreValue: Parse.Object = queryGenre;
9 const queryAuthorValue: Parse.Object = queryAuthor;
10 const queryIsbdValue: Parse.Object = queryIsbd;
11
12 // Reading parse objects is done by using Parse.Query
13 const parseQuery: Parse.Query = new Parse.Query('Book');
14
15 // One-to-many queries
16 if (queryPublisherValue !== '') {
17 parseQuery.equalTo('publisher', queryPublisherValue);
18 }
19 if (queryGenreValue !== '') {
20 parseQuery.equalTo('genre', queryGenreValue);
21 }
22
23 // One-to-one query
24 if (queryIsbdValue !== '') {
25 parseQuery.equalTo('isbd', queryIsbdValue);
26 }
27
28 // Many-to-many query
29 // In this case, we need to retrieve books related to the chosen author
30 if (queryAuthorValue !== '') {
31 parseQuery.equalTo('authors', queryAuthorValue);
32 }
33
34 try {
35 let books: [Parse.Object] = await parseQuery.find();
36 // Many-to-many objects retrieval
37 // In this example we need to get every related author Parse.Object
38 // and add it to our query result objects
39 for (let book of books) {
40 // This query is done by creating a relation and querying it
41 let bookAuthorsRelation = book.relation('authors');
42 book.authorsObjects = await bookAuthorsRelation.query().find();
43 }
44 setQueriedBooks(books);
45 return true;
46 } catch (error) {
47 // Error can be caused by lack of Internet connection
48 alert(`Error! ${error.message}`);
49 return false;
50 }
51 };One-to-many query
To query any books related to a specific publisher or genre, you need to perform a Parse.Query.equalTo method passing the Parse.Object instance as the parameter. After querying, Parse will store inside the resulting objects the complete instances of any one-to-many relational fields. To retrieve and show data from these object instances, you can chain the Parse.Object.get method like this: BookParseObject.get(('publisher').get('name').
Many-to-many query
To query any books related to a specific author, the query will also use only a Parse.Query.equalTo method. However, after querying, Parse will not store Parse.Object instances from many-to-many relational fields, only a reference to the related class name, such as {"__type": "Relation", "className": "Author"}. To retrieve and show data from these object instances, you need to create a relation and query it again, storing the results in an object array of your own.
One-to-one query
Just as before, to query any books related to a specific ISBD, you need to perform a Parse.Query.equalTo method passing the Parse.Object instance as the parameter. After querying, Parse will store inside the resulting objects the complete instances of any one-to-one relational fields, the same way that it is done with one-to-many relational fields.
Conclusion
At the end of this guide, you learned how to create and query relations in Parse on React. In the next guide, we will show you how to register users.