---
title: Relationships
slug: docs/android/data-objects/relationships
description: In this guide you'll learn how to create and query one-to-many or many-to-many data object associations in Parse on Android
image: https://www.back4app.com/images/logo-social.png
docTags:
createdAt: 2024-03-13T13:16:59.042Z
---
# Relationships on Android
## 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-one`, establishing direct relations between two objects and only them;
- `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.
There are two ways to create a `one-to-many` relation in Parse:
- The first is using the `Pointers` in `Child Class,` which is the fastest in creation and query time.
- The second is using `Arrays` of `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.
- The first is using the Parse `Relations`, which is the fastest in creation and query time. We will use this in this guide.
- The second is using `Arrays` of `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.
This tutorial uses a basic app created in Android Studio 4.1.1 with `buildToolsVersion=30.0.2` , `Compile SDK Version = 30.0.2` and `targetSdkVersion 30`
:::hint{type="success"}
At any time, you can access the complete Android Project built with this tutorial at our Github repositories
- [Kotlin Example Repository](https://github.com/templates-back4app/Android-Parse-Sdk-Kotlin)
- [Java Example Repository](https://github.com/templates-back4app/Android-Parse-Sdk-Java)
:::
## Goal
Our goal is, understand Parse Relations by creating a practical Book app.
Here is a preview of what we are gonna achieve:
::Image[]{src="https://api.archbee.com/api/optimize/yD3zCY-NNBBIfd0uqcfR5/4GItAcqwXy8pwNIso5U51_image.png" size="30" width="346" height="750" position="center" showCaption="false"}
## Prerequisites
:::hint{type="info"}
**To complete this tutorial, we need:**
- [Android Studio](https://developer.android.com/studio/index.html)
- An app created on Back4App.
- **Note: **Follow the [New Parse App tutorial](https://www.back4app.com/docs/get-started/new-parse-app) to learn how to create a Parse App on Back4App.
- An android app connected to Back4App.
- **Note: **Follow the [Install Parse SDK tutoria](https://www.back4app.com/docs/android/parse-android-sdk)l to create an Android Studio Project connected to Back4App.
- A device (or[ virtual device](https://developer.android.com/studio/run/managing-avds.html)) running Android 4.1 (Jelly Bean) or newer.
:::
## Understanding the Book App
The main object class you’ll be using is the `Book` class, storing each book entry in the registration. Also, these are the other three 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;
A visual representation of these data model:
## Let’s get started!
:::hint{type="info"}
Before next steps, we need to connect `Back4App` to our application. You should save the `appId` and `clientKey` from the `Back4App` to `string.xml` file and then init `Parse` in our `App.java` or `App.kt` file.
Follow the [New Parse App tutorial](https://www.back4app.com/docs/android/parse-android-sdk) if you don’t know how to init `Parse` to your app.
Or you can download the projects we shared the github links above and edit only the `appId` and `clientKey` parts according to you.
:::
## 1 - Save and list related objects of books
In this step we will see how to save and list the `Genres`, `Publishers` and `Authors` classes related with the `Book` class.
### 1.1 - Save and list Genres
We can register a`Genre`using the following snippet.
:::CodeblockTabs
```java
1 private void addGenre(String name) {
2 //We are taking this name parameter from the input.
3 progressDialog.show();
4 ParseObject parseObject = new ParseObject("Genre");
5 parseObject.put("name", name);
6 parseObject.saveInBackground(e -> {
7 progressDialog.dismiss();
8 if (e == null) {
9 getGenres();
10 inputGenre.setText("");
11 Toast.makeText(this, "Genre saved successfully", Toast.LENGTH_SHORT).show();
12 } else {
13 Toast.makeText(this, e.getLocalizedMessage(), Toast.LENGTH_SHORT).show();
14 }
15 });
16 }
```
```kotlin
1 private fun addGenre(name: String) {
2 //We are taking this name parameter from the input.
3 progressDialog.show()
4 val parseObject = ParseObject("Genre")
5 parseObject.put("name", name)
6 parseObject.saveInBackground {
7 progressDialog.dismiss()
8 if (it == null) {
9 getGenres()
10 inputGenre.setText("")
11 Toast.makeText(this, "Genre saved successfully", Toast.LENGTH_SHORT).show()
12 } else {
13 Toast.makeText(this, it.localizedMessage, Toast.LENGTH_SHORT).show()
14 }
15 }
16 }
```
:::
We can register a`Genre`using the following snippet.
:::CodeblockTabs
```java
1 private void getGenres() {
2 progressDialog.show();
3 ParseQuery query = new ParseQuery<>("Genre");
4 query.findInBackground((objects, e) -> {
5 progressDialog.dismiss();
6 List list = new ArrayList<>();
7 for (ParseObject parseObject : objects) {
8 list.add(new ParseObjectModel(parseObject));
9 }
10
11 GenreAdapter adapter = new GenreAdapter(list, this);
12 recyclerView.setLayoutManager(new LinearLayoutManager(this));
13 recyclerView.setAdapter(adapter);
14 });
15 }
```
```kotlin
1 private fun getGenres() {
2 progressDialog.show()
3 val query = ParseQuery("Genre")
4 query.findInBackground { objects, e ->
5 progressDialog.dismiss()
6 var list: MutableList = ArrayList()
7 for (parseObject in objects) {
8 list.add(ParseObjectModel(parseObject))
9 }
10 val adapter = GenreAdapter(this, list)
11 recyclerView.layoutManager = LinearLayoutManager(this)
12 recyclerView.adapter = adapter
13 }
14 }
```
:::
::Image[]{src="https://api.archbee.com/api/optimize/yD3zCY-NNBBIfd0uqcfR5/27F_Ufr36FnQWAFtLAzkg_image.png" size="30" width="532" height="1084" position="center" showCaption="false"}
### 1.2 - Save and list Publishers
We can register a`Publisher`using the following snippet.
:::CodeblockTabs
```java
1 private void addPublisher(String name) {
2 //We are taking this name parameter from the input.
3 progressDialog.show();
4 ParseObject parseObject = new ParseObject("Publisher");
5 parseObject.put("name", name);
6 parseObject.saveInBackground(e -> {
7 progressDialog.dismiss();
8 if (e == null) {
9 getPublishers();
10 inputPublisher.setText("");
11 Toast.makeText(this, "Publisher saved successfully", Toast.LENGTH_SHORT).show();
12 } else {
13 Toast.makeText(this, e.getLocalizedMessage(), Toast.LENGTH_SHORT).show();
14 }
15 });
16 }
```
```kotlin
1 private fun addPublisher(name: String) {
2 //We are taking this name parameter from the input.
3 progressDialog.show()
4 val parseObject = ParseObject("Publisher")
5 parseObject.put("name", name)
6 parseObject.saveInBackground {
7 progressDialog.dismiss()
8 if (it == null) {
9 getPublishers()
10 inputPublisher.setText("")
11 Toast.makeText(this, "Publisher saved successfully", Toast.LENGTH_SHORT).show()
12 } else {
13 Toast.makeText(this, it.localizedMessage, Toast.LENGTH_SHORT).show()
14 }
15 }
16 }
```
:::
We can register a`Publisher`using the following snippet.
:::CodeblockTabs
```java
1 private void getPublishers() {
2 progressDialog.show();
3 ParseQuery query = new ParseQuery<>("Publisher");
4 query.findInBackground((objects, e) -> {
5 progressDialog.dismiss();
6 List list = new ArrayList<>();
7 for (ParseObject parseObject : objects) {
8 list.add(new ParseObjectModel(parseObject));
9 }
10
11 PublisherAdapter adapter = new PublisherAdapter(list, this);
12 recyclerView.setLayoutManager(new LinearLayoutManager(this));
13 recyclerView.setAdapter(adapter);
14 });
15 }
```
```kotlin
1 private fun getPublishers() {
2 progressDialog.show()
3 val query = ParseQuery("Publisher")
4 query.findInBackground { objects, e ->
5 progressDialog.dismiss()
6 val list: ArrayList = ArrayList()
7 for (parseObject in objects) {
8 list.add(ParseObjectModel(parseObject))
9 }
10
11 val adapter = PublisherAdapter(this, list)
12 recyclerView.layoutManager = LinearLayoutManager(this)
13 recyclerView.adapter = adapter
14 }
15 }
```
:::
::Image[]{src="https://api.archbee.com/api/optimize/yD3zCY-NNBBIfd0uqcfR5/As2skdoT9UlsTddqEfirE_image.png" size="30" width="532" height="1084" position="center" showCaption="false"}
### 1.3 - Save and list Authors
We can register a`Author`using the following snippet.
:::CodeblockTabs
```java
1 private void addAuthor(String name){
2 //We are taking this name parameter from the input.
3 progressDialog.show();
4 ParseObject parseObject = new ParseObject("Author");
5 parseObject.put("name", name);
6 parseObject.saveInBackground(e -> {
7 progressDialog.dismiss();
8 if (e == null) {
9 getAuthors();
10 inputAuthor.setText("");
11 Toast.makeText(this, "Author saved successfully", Toast.LENGTH_SHORT).show();
12 } else {
13 Toast.makeText(this, e.getLocalizedMessage(), Toast.LENGTH_SHORT).show();
14 }
15 });
16 }
```
```kotlin
1 private fun addAuthor(name: String) {
2 //We are taking this name parameter from the input.
3 progressDialog.show()
4 val parseObject = ParseObject("Author")
5 parseObject.put("name", name)
6 parseObject.saveInBackground {
7 progressDialog.dismiss()
8 if (it == null) {
9 getAuthors()
10 inputAuthor.setText("")
11 Toast.makeText(this, "Author saved successfully", Toast.LENGTH_SHORT).show()
12 } else {
13 Toast.makeText(this, it.localizedMessage, Toast.LENGTH_SHORT).show()
14 }
15 }
16 }
```
:::
We can register a`Author`using the following snippet.
:::CodeblockTabs
```java
1 private void getAuthors() {
2 progressDialog.show();
3 ParseQuery query = new ParseQuery<>("Author");
4 query.findInBackground((objects, e) -> {
5 progressDialog.dismiss();
6 List list = new ArrayList<>();
7 for (ParseObject parseObject : objects) {
8 list.add(new ParseObjectModel(parseObject));
9 }
10
11 AuthorAdapter adapter = new AuthorAdapter(list, this);
12 recyclerView.setLayoutManager(new LinearLayoutManager(this));
13 recyclerView.setAdapter(adapter);
14 });
15 }
```
```kotlin
1 private fun getAuthors() {
2 progressDialog.show()
3 val query = ParseQuery("Author")
4 query.findInBackground { objects: List, e: ParseException? ->
5 progressDialog.dismiss()
6 val list: ArrayList = ArrayList()
7 for (parseObject in objects) {
8 list.add(ParseObjectModel(parseObject))
9 }
10 val adapter = AuthorAdapter(this, list)
11 recyclerView.layoutManager = LinearLayoutManager(this)
12 recyclerView.adapter = adapter
13 }
14 }
```
:::
::Image[]{src="https://api.archbee.com/api/optimize/yD3zCY-NNBBIfd0uqcfR5/KNTZxKCHMAjZAKFGtW9j5_image.png" size="30" width="532" height="1084" position="center" showCaption="false"}
In this part, we use a model class named `ParseObjectModel`. In this model class, we have a `ParseObject` variable to be able to read the data, and the `isChecked` variable, which we will use to save the book in the next step. We will be able to easily retrieve the selected objects with the `isChecked` variable.
Here is the our `ParseObjectModel` model.
:::CodeblockTabs
```java
1 public class ParseObjectModel {
2 ParseObject object;
3 boolean isChecked = false;
4
5 public ParseObjectModel(ParseObject object) {
6 this.object = object;
7 }
8
9 public ParseObject getObject() {
10 return object;
11 }
12
13 public ParseObjectModel setObject(ParseObject object) {
14 this.object = object;
15 return this;
16 }
17
18 public boolean isChecked() {
19 return isChecked;
20 }
21
22 public ParseObjectModel setChecked(boolean checked) {
23 isChecked = checked;
24 return this;
25 }
26 }
```
```kotlin
1 class ParseObjectModel(obj: ParseObject) {
2 var obj: ParseObject? = null
3 var isChecked: Boolean = false
4
5 init {
6 this.obj = obj
7 }
8 }
```
:::
## 2 - Save a Book object and its relations
### 2.1 - Save a Book object with 1\:N Relationship
This function will create a new `Book` in Back4app database with 1\:N relations.
:::CodeblockTabs
```java
1 progressDialog.show();
2 book.put("genre", genre);
3 book.put("publisher", publisher);
4 book.put("title", title);
5 book.put("year", year);
6 book.saveInBackground(e -> {
7 progressDialog.hide();
8 if (e == null) {
9 Toast.makeText(AddBookActivity.this, "Book saved successfully", Toast.LENGTH_SHORT).show();
10 startActivity(new Intent(AddBookActivity.this, BookListActivity.class));
11 finish();
12 } else {
13 Toast.makeText(AddBookActivity.this, e.getLocalizedMessage(), Toast.LENGTH_SHORT).show();
14 }
15 });
```
```kotlin
1 progressDialog.show()
2 book.put("genre", genre)
3 book.put("publisher", publisher!!)
4 book.put("title", title)
5 book.put("year", year)
6 book.saveInBackground {
7 progressDialog.hide()
8 if (it == null) {
9 Toast.makeText(this, "Book saved successfully", Toast.LENGTH_SHORT).show()
10 startActivity(Intent(this@AddBookActivity, BookListActivity::class.java))
11 finish()
12 } else {
13 Toast.makeText(this, it.localizedMessage, Toast.LENGTH_SHORT).show()
14 }
15 }
```
:::
### 2.2 Save a Book object with N\:N Relationship
This function will create a new `Book` in Back4app database with N\:N relations. For the `Author` relation, we find the selected Author/s in the adapter of the `authorRecyclerView` and save them as `Parse Relation`.
:::CodeblockTabs
```java
1 progressDialog.show();
2 book.put("genre", genre);
3 book.put("publisher", publisher);
4 book.put("title", title);
5 book.put("year", year);
6
7 //Here we are setting book relation with getSelectedItem function of BookAuthorAdapter.
8 if (recyclerViewAuthors.getAdapter() != null) {
9 relation = ((BookAuthorAdapter) recyclerViewAuthors.getAdapter()).getSelectedItems(book);
10 if (relation == null) {
11 Toast.makeText(this, "Please select Author/s", Toast.LENGTH_SHORT).show();
12 return;
13 }
14 } else {
15 Toast.makeText(this, "Something went wrong!!", Toast.LENGTH_SHORT).show();
16 return;
17 }
18
19 book.saveInBackground(e -> {
20 progressDialog.hide();
21 if (e == null) {
22 Toast.makeText(AddBookActivity.this, "Book saved successfully", Toast.LENGTH_SHORT).show();
23 startActivity(new Intent(AddBookActivity.this, BookListActivity.class));
24 finish();
25 } else {
26 Toast.makeText(AddBookActivity.this, e.getLocalizedMessage(), Toast.LENGTH_SHORT).show();
27 }
28 });
29
30 //This is the function for save Author/s relation of Book object.This function in BookAuthorAdapter.
31 public ParseRelation getSelectedItems(ParseObject parseObject) {
32 ParseRelation relation = parseObject.getRelation("author_relation");
33 for (ParseObjectModel object : this.list) {
34 if (object.isChecked())
35 relation.add(object.getObject());
36 }
37 return relation;
38 }
```
```kotlin
1 progressDialog.show()
2 book.put("genre", genre)
3 book.put("publisher", publisher!!)
4 book.put("title", title)
5 book.put("year", year)
6
7 //Here we are setting book relation with getSelectedItem function of BookAuthorAdapter.
8
9 if (recyclerViewAuthors.adapter != null) {
10 relation = (recyclerViewAuthors.adapter as BookAuthorAdapter).getSelectedItems(book)
11 if (relation == null) {
12 Toast.makeText(this, "Please select Author/s", Toast.LENGTH_SHORT).show()
13 return
14 }
15 } else {
16 Toast.makeText(this, "Something went wrong!!", Toast.LENGTH_SHORT).show()
17 return
18 }
19
20 book.saveInBackground {
21 progressDialog.hide()
22 if (it == null) {
23 Toast.makeText(this, "Book saved successfully", Toast.LENGTH_SHORT).show()
24 startActivity(Intent(this@AddBookActivity, BookListActivity::class.java))
25 finish()
26 } else {
27 Toast.makeText(this, it.localizedMessage, Toast.LENGTH_SHORT).show()
28 }
29 }
30
31 //This is the function for save Author/s relation of Book object.This function in BookAuthorAdapter.
32
33 fun getSelectedItems(parseObject: ParseObject): ParseRelation? {
34 var relation:ParseRelation? = parseObject.getRelation("author_relation")
35 for (obj in this.list) {
36 if (obj.isChecked)
37 relation?.add(obj.obj)
38 }
39 return relation
40 }
```
:::
## 3 - Query the Book Details with Relations
With these functions, we will list our `Books` according to their `Publishers`. First, we throw a query to the Publisher class.
:::CodeblockTabs
```java
1 progressDialog.show();
2 ParseQuery query = new ParseQuery<>("Publisher");
3 query.findInBackground((objects, e) -> {
4 progressDialog.hide();
5 if (e == null) {
6 BookListAdapter adapter = new BookListAdapter(objects, this);
7 recyclerView.setLayoutManager(new LinearLayoutManager(this));
8 recyclerView.setAdapter(adapter);
9 } else {
10 Toast.makeText(this, e.getLocalizedMessage(), Toast.LENGTH_SHORT).show();
11 }
12 });
```
```kotlin
1 progressDialog.show()
2 val query = ParseQuery("Publisher")
3 query.findInBackground { objects: List?, e: ParseException? ->
4 progressDialog.hide()
5 if (e == null) {
6 val adapter = BookListAdapter(this, objects!!)
7 recyclerView.layoutManager = LinearLayoutManager(this)
8 recyclerView.adapter = adapter
9 } else {
10 Toast.makeText(this, e.localizedMessage, Toast.LENGTH_SHORT).show()
11 }
12 }
```
:::
And then we query to list the `Books` that each `Publisher` item is related to.
:::CodeblockTabs
```java
1 ParseObject object = list.get(position);
2 holder.title.setText(object.getString("name"));
3 ParseQuery query = new ParseQuery<>("Book");
4 query.whereEqualTo("publisher", object);
5 query.findInBackground((objects, e) -> {
6 if (e == null) {
7 BooksAdapter adapter = new BooksAdapter(objects, context);
8 holder.books.setLayoutManager(new LinearLayoutManager(context));
9 holder.books.setAdapter(adapter);
10 } else {
11 Toast.makeText(context, e.getLocalizedMessage(), Toast.LENGTH_SHORT).show();
12 }
13 });
```
```kotlin
1 val `object` = list[position]
2 holder.title.text = `object`.getString("name")
3 val query = ParseQuery("Book")
4 query.whereEqualTo("publisher", `object`)
5 query.findInBackground { objects: List?, e: ParseException? ->
6 if (e == null) {
7 val adapter = BooksAdapter(context, objects!!)
8 holder.books.layoutManager = LinearLayoutManager(context)
9 holder.books.adapter = adapter
10 } else {
11 Toast.makeText(context, e.localizedMessage, Toast.LENGTH_SHORT).show()
12 }
13 }
```
:::
::Image[]{src="https://api.archbee.com/api/optimize/yD3zCY-NNBBIfd0uqcfR5/_mdZWSy6L--bhqBCF_Dz3_image.png" size="30" width="532" height="1084" position="center" showCaption="false"}
Now, when we click on any `Book` object, we send the Object Id of this `Book` with an intent to the page that will show the details of that `Book`. And we get all the details of the `Book` from the database by using this Object Id on that page.
:::CodeblockTabs
```java
1 private void getBookWithDetails() {
2 progressDialog.show();
3 ParseQuery query = new ParseQuery<>("Book");
4 query.getInBackground(getIntent().getStringExtra("objectId"), (object, e) -> {
5 if (e == null) {
6 bookTitle.setText("Title: " +object.getString("title"));
7 bookYear.setText("Year: " +object.getString("year"));
8 try {
9 bookGenre.setText("Genre: " +object.getParseObject("genre").fetchIfNeeded().getString("name"));
10 } catch (ParseException parseException) {
11 parseException.printStackTrace();
12 }
13 try {
14 bookPublisher.setText("Publisher: " + object.getParseObject("publisher").fetchIfNeeded().getString("name"));
15 } catch (ParseException parseException) {
16 parseException.printStackTrace();
17 }
18
19 object.getRelation("author_relation").getQuery().findInBackground((objects, e1) -> {
20 progressDialog.hide();
21 if (e1 == null) {
22 BookDetailAuthorAdapter adapter = new BookDetailAuthorAdapter(objects, this);
23 authorRecyclerView.setLayoutManager(new LinearLayoutManager(this));
24 authorRecyclerView.setAdapter(adapter);
25 } else {
26 Toast.makeText(this, e.getLocalizedMessage(), Toast.LENGTH_SHORT).show();
27 }
28 });
29 } else {
30 progressDialog.hide();
31 Toast.makeText(this, e.getLocalizedMessage(), Toast.LENGTH_SHORT).show();
32 }
33 });
34 }
```
```kotlin
1 private fun getBookWithDetails() {
2 progressDialog.show()
3 val query = ParseQuery("Book")
4
5 query.getInBackground(intent.getStringExtra("objectId")) { `object`, e ->
6 if (e == null) {
7 bookTitle.text = "Title: " + `object`.getString("title")
8 bookYear.text = "Year: " + `object`.getString("year")
9 try {
10 bookGenre.text = "Genre: " + `object`.getParseObject("genre")?.fetchIfNeeded()?.getString("name")
11 } catch (parseException: ParseException) {
12 parseException.printStackTrace()
13 }
14 try {
15 bookPublisher.text =
16 "Publisher: " + `object`.getParseObject("publisher")?.fetchIfNeeded()?.getString("name")
17 } catch (parseException: ParseException) {
18 parseException.printStackTrace()
19 }
20
21 `object`.getRelation("author_relation").query.findInBackground { objects, e1 ->
22 progressDialog.hide()
23 if (e1 == null) {
24 val adapter = BookDetailAuthorAdapter(this, objects)
25 authorRecyclerView.layoutManager = LinearLayoutManager(this)
26 authorRecyclerView.adapter = adapter
27 } else {
28 Toast.makeText(this, e1.localizedMessage, Toast.LENGTH_SHORT).show()
29 }
30 }
31 } else {
32 progressDialog.hide()
33 Toast.makeText(this, e.localizedMessage, Toast.LENGTH_SHORT).show()
34 }
35 }
36 }
```
:::
## It’s done!
At this point, we have learned `Parse Relationships` on `Android`.