@rikishi/watermelondb
Version:
Build powerful React Native and React web apps that scale from hundreds to tens of thousands of records and remain fast
369 lines (280 loc) • 20.1 kB
HTML
<html lang="en" class="sidebar-visible no-js light">
<head>
<!-- Book generated using mdBook -->
<meta charset="UTF-8">
<title>Relations - WatermelonDB documentation</title>
<!-- Custom HTML head -->
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
<meta name="description" content="">
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta name="theme-color" content="#ffffff" />
<link rel="icon" href="favicon.svg">
<link rel="shortcut icon" href="favicon.png">
<link rel="stylesheet" href="css/variables.css">
<link rel="stylesheet" href="css/general.css">
<link rel="stylesheet" href="css/chrome.css">
<link rel="stylesheet" href="css/print.css" media="print">
<!-- Fonts -->
<link rel="stylesheet" href="FontAwesome/css/font-awesome.css">
<link rel="stylesheet" href="fonts/fonts.css">
<!-- Highlight.js Stylesheets -->
<link rel="stylesheet" href="highlight.css">
<link rel="stylesheet" href="tomorrow-night.css">
<link rel="stylesheet" href="ayu-highlight.css">
<!-- Custom theme stylesheets -->
</head>
<body>
<!-- Provide site root to javascript -->
<script type="text/javascript">
var path_to_root = "";
var default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? "navy" : "light";
</script>
<!-- Work around some values being stored in localStorage wrapped in quotes -->
<script type="text/javascript">
try {
var theme = localStorage.getItem('mdbook-theme');
var sidebar = localStorage.getItem('mdbook-sidebar');
if (theme.startsWith('"') && theme.endsWith('"')) {
localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1));
}
if (sidebar.startsWith('"') && sidebar.endsWith('"')) {
localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1));
}
} catch (e) { }
</script>
<!-- Set the theme before any content is loaded, prevents flash -->
<script type="text/javascript">
var theme;
try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
if (theme === null || theme === undefined) { theme = default_theme; }
var html = document.querySelector('html');
html.classList.remove('no-js')
html.classList.remove('light')
html.classList.add(theme);
html.classList.add('js');
</script>
<!-- Hide / unhide sidebar before it is displayed -->
<script type="text/javascript">
var html = document.querySelector('html');
var sidebar = 'hidden';
if (document.body.clientWidth >= 1080) {
try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
sidebar = sidebar || 'visible';
}
html.classList.remove('sidebar-visible');
html.classList.add("sidebar-" + sidebar);
</script>
<nav id="sidebar" class="sidebar" aria-label="Table of contents">
<div class="sidebar-scrollbox">
<ol class="chapter"><li class="chapter-item expanded "><a href="ch01-00-get-excited.html"><strong aria-hidden="true">1.</strong> Get excited</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="index.html"><strong aria-hidden="true">1.1.</strong> Check out the README</a></li><li class="chapter-item expanded "><a href="Demo.html"><strong aria-hidden="true">1.2.</strong> See the demo</a></li></ol></li><li class="chapter-item expanded "><a href="ch02-00-learn-to-use.html"><strong aria-hidden="true">2.</strong> Learn to use Watermelon</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="Installation.html"><strong aria-hidden="true">2.1.</strong> Installation</a></li><li class="chapter-item expanded "><a href="Setup.html"><strong aria-hidden="true">2.2.</strong> Setup</a></li><li class="chapter-item expanded "><a href="Schema.html"><strong aria-hidden="true">2.3.</strong> Schema</a></li><li class="chapter-item expanded "><a href="Model.html"><strong aria-hidden="true">2.4.</strong> Defining Models</a></li><li class="chapter-item expanded "><a href="CRUD.html"><strong aria-hidden="true">2.5.</strong> Create, Read, Update, Delete</a></li><li class="chapter-item expanded "><a href="Components.html"><strong aria-hidden="true">2.6.</strong> Connecting to React Components</a></li><li class="chapter-item expanded "><a href="Query.html"><strong aria-hidden="true">2.7.</strong> Querying</a></li><li class="chapter-item expanded "><a href="Relation.html" class="active"><strong aria-hidden="true">2.8.</strong> Relations</a></li><li class="chapter-item expanded "><a href="Writers.html"><strong aria-hidden="true">2.9.</strong> Writers, Readers, batching</a></li></ol></li><li class="chapter-item expanded "><a href="ch03-00-advanced.html"><strong aria-hidden="true">3.</strong> Advanced guides</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="Advanced/Migrations.html"><strong aria-hidden="true">3.1.</strong> Migrations</a></li><li class="chapter-item expanded "><a href="Advanced/Sync.html"><strong aria-hidden="true">3.2.</strong> Sync</a></li><li class="chapter-item expanded "><a href="Advanced/CreateUpdateTracking.html"><strong aria-hidden="true">3.3.</strong> Automatic create/update tracking</a></li><li class="chapter-item expanded "><a href="Advanced/AdvancedFields.html"><strong aria-hidden="true">3.4.</strong> Advanced fields</a></li><li class="chapter-item expanded "><a href="Advanced/Flow.html"><strong aria-hidden="true">3.5.</strong> Flow</a></li><li class="chapter-item expanded "><a href="Advanced/LocalStorage.html"><strong aria-hidden="true">3.6.</strong> LocalStorage</a></li><li class="chapter-item expanded "><a href="Advanced/ProTips.html"><strong aria-hidden="true">3.7.</strong> Pro tips</a></li><li class="chapter-item expanded "><a href="Advanced/Performance.html"><strong aria-hidden="true">3.8.</strong> Performance tips</a></li><li class="chapter-item expanded "><a href="Advanced/SharingDatabaseAcrossTargets.html"><strong aria-hidden="true">3.9.</strong> iOS - Sharing database across targets</a></li></ol></li><li class="chapter-item expanded "><a href="ch04-00-deeper.html"><strong aria-hidden="true">4.</strong> Dig deeper into WatermelonDB</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="Implementation/Architecture.html"><strong aria-hidden="true">4.1.</strong> Architecture</a></li><li class="chapter-item expanded "><a href="Implementation/Adapters.html"><strong aria-hidden="true">4.2.</strong> Adapters</a></li><li class="chapter-item expanded "><a href="Implementation/SyncImpl.html"><strong aria-hidden="true">4.3.</strong> Sync implementation</a></li></ol></li><li class="chapter-item expanded "><a href="ch04-00-deeper.html"><strong aria-hidden="true">5.</strong> Other</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="Roadmap.html"><strong aria-hidden="true">5.1.</strong> Roadmap</a></li><li class="chapter-item expanded "><a href="CONTRIBUTING.html"><strong aria-hidden="true">5.2.</strong> Contributing</a></li><li class="chapter-item expanded "><a href="CHANGELOG.html"><strong aria-hidden="true">5.3.</strong> Changelog</a></li></ol></li></ol>
</div>
<div id="sidebar-resize-handle" class="sidebar-resize-handle"></div>
</nav>
<div id="page-wrapper" class="page-wrapper">
<div class="page">
<div id="menu-bar-hover-placeholder"></div>
<div id="menu-bar" class="menu-bar sticky bordered">
<div class="left-buttons">
<button id="sidebar-toggle" class="icon-button" type="button" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar">
<i class="fa fa-bars"></i>
</button>
<button id="theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="theme-list">
<i class="fa fa-paint-brush"></i>
</button>
<ul id="theme-list" class="theme-popup" aria-label="Themes" role="menu">
<li role="none"><button role="menuitem" class="theme" id="light">Light (default)</button></li>
<li role="none"><button role="menuitem" class="theme" id="rust">Rust</button></li>
<li role="none"><button role="menuitem" class="theme" id="coal">Coal</button></li>
<li role="none"><button role="menuitem" class="theme" id="navy">Navy</button></li>
<li role="none"><button role="menuitem" class="theme" id="ayu">Ayu</button></li>
</ul>
<button id="search-toggle" class="icon-button" type="button" title="Search. (Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar">
<i class="fa fa-search"></i>
</button>
</div>
<h1 class="menu-title">WatermelonDB documentation</h1>
<div class="right-buttons">
<a href="print.html" title="Print this book" aria-label="Print this book">
<i id="print-button" class="fa fa-print"></i>
</a>
</div>
</div>
<div id="search-wrapper" class="hidden">
<form id="searchbar-outer" class="searchbar-outer">
<input type="search" name="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header">
</form>
<div id="searchresults-outer" class="searchresults-outer hidden">
<div id="searchresults-header" class="searchresults-header"></div>
<ul id="searchresults">
</ul>
</div>
</div>
<!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM -->
<script type="text/javascript">
document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible');
document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible');
Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) {
link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1);
});
</script>
<div id="content" class="content">
<main>
<h1 id="relations"><a class="header" href="#relations">Relations</a></h1>
<p>A <code>Relation</code> object represents one record pointing to another — such as the author (<code>User</code>) of a <code>Comment</code>, or the <code>Post</code> the comment belongs to.</p>
<h3 id="defining-relations"><a class="header" href="#defining-relations">Defining Relations</a></h3>
<p>There's two steps to defining a relation:</p>
<ol>
<li>
<p>A <a href="./Schema.html"><strong>table column</strong></a> for the related record's ID</p>
<pre><code class="language-js">tableSchema({
name: 'comments',
columns: [
// ...
{ name: 'author_id', type: 'string' },
]
}),
</code></pre>
</li>
<li>
<p>A <code>@relation</code> field <a href="./Model.html">defined on a <code>Model</code></a> class:</p>
<pre><code class="language-js">import { relation } from '@rikishi/watermelondb/decorators'
class Comment extends Model {
// ...
@relation('users', 'author_id') author
}
</code></pre>
<p>The first argument is the <em>table name</em> of the related record, and the second is the <em>column name</em> with an ID for the related record.</p>
</li>
</ol>
<h3 id="immutablerelation"><a class="header" href="#immutablerelation">immutableRelation</a></h3>
<p>If you have a relation that cannot change (for example, a comment can't change its author), use <code>@immutableRelation</code> for extra protection and performance:</p>
<pre><code class="language-js">import { immutableRelation } from '@rikishi/watermelondb/decorators'
class Comment extends Model {
// ...
@immutableRelation('posts', 'post_id') post
@immutableRelation('users', 'author_id') author
}
</code></pre>
<h2 id="relation-api"><a class="header" href="#relation-api">Relation API</a></h2>
<p>In the example above, <code>comment.author</code> returns a <code>Relation</code> object.</p>
<blockquote>
<p>Remember, WatermelonDB is a lazily-loaded database, so you don't get the related <code>User</code> record immediately, only when you explicitly fetch it</p>
</blockquote>
<h3 id="observing"><a class="header" href="#observing">Observing</a></h3>
<p>Most of the time, you <a href="./Components.html">connect Relations to Components</a> by using <code>observe()</code> (the same <a href="./Query.html">as with Queries</a>):</p>
<pre><code class="language-js">withObservables(['comment'], ({ comment }) => ({
comment,
author: comment.author, // shortcut syntax for `author: comment.author.observe()`
}))
</code></pre>
<p>The component will now have an <code>author</code> prop containing a <code>User</code>, and will re-render both when the user changes (e.g. comment's author changes its name), but also when a new author is assigned to the comment (if that was possible).</p>
<h3 id="fetching"><a class="header" href="#fetching">Fetching</a></h3>
<p>To simply get the related record, use <code>fetch</code>. You might need it <a href="./Writers.html">in a Writer</a></p>
<pre><code class="language-js">const author = await comment.author.fetch()
// Shortcut syntax:
const author = await comment.author
</code></pre>
<p><strong>Note</strong>: If the relation column (in this example, <code>author_id</code>) is marked as <code>isOptional: true</code>, <code>fetch()</code> might return <code>null</code>.</p>
<h3 id="id"><a class="header" href="#id">ID</a></h3>
<p>If you only need the ID of a related record (e.g. to use in an URL or for the <code>key=</code> React prop), use <code>id</code>.</p>
<pre><code class="language-js">const authorId = comment.author.id
</code></pre>
<h3 id="assigning"><a class="header" href="#assigning">Assigning</a></h3>
<p>Use <code>set()</code> to assign a new record to the relation</p>
<pre><code class="language-js">await database.get('comments').create(comment => {
comment.author.set(someUser)
// ...
})
</code></pre>
<p><strong>Note</strong>: you can only do this in the <code>.create()</code> or <code>.update()</code> block.</p>
<p>You can also use <code>set id</code> if you only have the ID for the record to assign</p>
<pre><code class="language-js">await comment.update(() => {
comment.author.id = userId
})
</code></pre>
<h2 id="advanced-relations"><a class="header" href="#advanced-relations">Advanced relations</a></h2>
<h3 id="many-to-many-relation"><a class="header" href="#many-to-many-relation">Many-To-Many Relation</a></h3>
<p>If for instance, our app <code>Post</code>s can be authored by many <code>User</code>s and a user can author many <code>Post</code>s. We would create such a relation following these steps:-</p>
<ol>
<li>Create a pivot schema and model that both the <code>User</code> model and <code>Post</code> model has association to; say <code>PostAuthor</code></li>
<li>Create has_many association on both <code>User</code> and <code>Post</code> pointing to <code>PostAuthor</code> Model</li>
<li>Create belongs_to association on <code>PostAuthor</code> pointing to both <code>User</code> and <code>Post</code></li>
<li>Retrieve all <code>Posts</code> for a user by defining a query that uses the pivot <code>PostAuthor</code> to infer the <code>Post</code>s that were authored by the User.</li>
</ol>
<pre><code class="language-js">import { lazy } from '@rikishi/watermelondb/decorators'
class Post extends Model {
static table = 'posts'
static associations = {
post_authors: { type: 'has_many', foreignKey: 'post_id' },
}
@lazy
authors = this.collections
.get('users')
.query(Q.on('post_authors', 'post_id', this.id));
}
</code></pre>
<pre><code class="language-js">import { field } from '@rikishi/watermelondb/decorators'
class PostAuthor extends Model {
static table = 'post_authors'
static associations = {
posts: { type: 'belongs_to', key: 'post_id' },
users: { type: 'belongs_to', key: 'user_id' },
}
@immutableRelation('posts', 'post_id') post
@immutableRelation('users', 'user_id') user
}
</code></pre>
<pre><code class="language-js">import { lazy } from '@rikishi/watermelondb/decorators'
class User extends Model {
static table = 'users'
static associations = {
post_authors: { type: 'has_many', foreignKey: 'user_id' },
}
@lazy
posts = this.collections
.get('posts')
.query(Q.on('post_authors', 'user_id', this.id));
}
</code></pre>
<pre><code class="language-js">withObservables(['post'], ({ post }) => ({
authors: post.authors,
}))
</code></pre>
<hr />
<h2 id="next-steps"><a class="header" href="#next-steps">Next steps</a></h2>
<p>➡️ Now the last step of this guide: <a href="./Writers.html"><strong>understand Writers (and Readers)</strong></a></p>
</main>
<nav class="nav-wrapper" aria-label="Page navigation">
<!-- Mobile navigation buttons -->
<a rel="prev" href="Query.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
<i class="fa fa-angle-left"></i>
</a>
<a rel="next" href="Writers.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
<i class="fa fa-angle-right"></i>
</a>
<div style="clear: both"></div>
</nav>
</div>
</div>
<nav class="nav-wide-wrapper" aria-label="Page navigation">
<a rel="prev" href="Query.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
<i class="fa fa-angle-left"></i>
</a>
<a rel="next" href="Writers.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
<i class="fa fa-angle-right"></i>
</a>
</nav>
</div>
<script type="text/javascript">
window.playground_copyable = true;
</script>
<script src="elasticlunr.min.js" type="text/javascript" charset="utf-8"></script>
<script src="mark.min.js" type="text/javascript" charset="utf-8"></script>
<script src="searcher.js" type="text/javascript" charset="utf-8"></script>
<script src="clipboard.min.js" type="text/javascript" charset="utf-8"></script>
<script src="highlight.js" type="text/javascript" charset="utf-8"></script>
<script src="book.js" type="text/javascript" charset="utf-8"></script>
<!-- Custom JS scripts -->
</body>
</html>