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<ol class="chapter"><li class="chapter-item expanded "><a href="index.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li class="chapter-item expanded "><a href="installation_features.html"><strong aria-hidden="true">2.</strong> Installation and Features</a></li><li class="chapter-item expanded "><a href="connecting.html"><strong aria-hidden="true">3.</strong> Connecting to the Database</a></li><li class="chapter-item expanded "><a href="reading.html" class="active"><strong aria-hidden="true">4.</strong> Reading From the Database</a></li><li class="chapter-item expanded "><div><strong aria-hidden="true">5.</strong> Writing To the Database</div></li><li class="chapter-item expanded "><a href="performance.html"><strong aria-hidden="true">6.</strong> Performance</a></li><li class="chapter-item expanded "><div><strong aria-hidden="true">7.</strong> Serde Integration</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">8.</strong> Sessions and Transactions</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">9.</strong> Change Streams</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">10.</strong> Monitoring</div></li><li class="chapter-item expanded "><a href="tracing.html"><strong aria-hidden="true">11.</strong> Tracing and Logging</a></li><li class="chapter-item expanded "><a href="web_framework_examples.html"><strong aria-hidden="true">12.</strong> Web Framework Examples</a></li><li class="chapter-item expanded "><a href="encryption.html"><strong aria-hidden="true">13.</strong> Encryption</a></li><li class="chapter-item expanded affix "><li class="part-title">Development</li><li class="chapter-item expanded "><div><strong aria-hidden="true">14.</strong> Writing Tests</div></li></ol>
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<h1 id="reading-from-the-database"><a class="header" href="#reading-from-the-database">Reading From the Database</a></h1>
<h2 id="database-and-collection-handles"><a class="header" href="#database-and-collection-handles">Database and Collection Handles</a></h2>
<p>Once you have a <code>Client</code>, you can call <a href="https://docs.rs/mongodb/latest/mongodb/struct.Client.html#method.database"><code>Client::database</code></a> to create a handle to a particular database on the server, and <a href="https://docs.rs/mongodb/latest/mongodb/struct.Database.html#method.collection"><code>Database::collection</code></a> to create a handle to a particular collection in that database. <a href="https://docs.rs/mongodb/latest/mongodb/struct.Database.html"><code>Database</code></a> and <a href="https://docs.rs/mongodb/latest/mongodb/struct.Collection.html"><code>Collection</code></a> handles are lightweight - creating them requires no IO, <code>clone</code>ing them is cheap, and they can be safely shared across threads or async tasks. For example:</p>
<pre><pre class="playground"><code class="language-rust no_run edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span><span class="boring">extern crate mongodb;
</span><span class="boring">extern crate tokio;
</span><span class="boring">use mongodb::{bson::Document, Client, error::Result};
</span><span class="boring">use tokio::task;
</span><span class="boring">
</span><span class="boring">async fn start_workers() -> Result<()> {
</span><span class="boring">let client = Client::with_uri_str("mongodb://example.com").await?;
</span>let db = client.database("items");
for i in 0..5 {
let db_ref = db.clone();
task::spawn(async move {
let collection = db_ref.collection::<Document>(&format!("coll{}", i));
// Do something with the collection
});
}
<span class="boring">
</span><span class="boring">Ok(())
</span><span class="boring">}
</span><span class="boring">}</span></code></pre></pre>
<p>A <code>Collection</code> can be parameterized with a type for the documents in the collection; this includes but is not limited to just <code>Document</code>. The various methods that accept instances of the documents (e.g. <a href="https://docs.rs/mongodb/latest/mongodb/struct.Collection.html#method.insert_one"><code>Collection::insert_one</code></a>) require that it implement the <code>Serialize</code> trait from the <a href="http://serde.rs/"><code>serde</code></a> crate. Similarly, the methods that return instances (e.g. <a href="https://docs.rs/mongodb/latest/mongodb/struct.Collection.html#method.find_one"><code>Collection::find_one</code></a>) require that it implement <code>Deserialize</code>.</p>
<p><code>Document</code> implements both and can always be used as the type parameter. However, it is recommended to define types that model your data which you can parameterize your <code>Collection</code>s with instead, since doing so eliminates a lot of boilerplate deserialization code and is often more performant.</p>
<pre><pre class="playground"><code class="language-rust no_run edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span><span class="boring">extern crate mongodb;
</span><span class="boring">extern crate tokio;
</span><span class="boring">extern crate serde;
</span><span class="boring">use mongodb::{
</span><span class="boring"> bson::doc,
</span><span class="boring"> error::Result,
</span><span class="boring">};
</span><span class="boring">use tokio::task;
</span><span class="boring">
</span><span class="boring">async fn start_workers() -> Result<()> {
</span><span class="boring">use mongodb::Client;
</span><span class="boring">
</span><span class="boring">let client = Client::with_uri_str("mongodb://example.com").await?;
</span>use serde::{Deserialize, Serialize};
// Define a type that models our data.
#[derive(Clone, Debug, Deserialize, Serialize)]
struct Item {
id: u32,
}
// Parameterize our collection with the model.
let coll = client.database("items").collection::<Item>("in_stock");
for i in 0..5 {
// Perform operations that work with directly our model.
coll.insert_one(Item { id: i }, None).await;
}
<span class="boring">
</span><span class="boring">Ok(())
</span><span class="boring">}
</span><span class="boring">}</span></code></pre></pre>
<p>For more information, see the <a href="serde_integration.html">Serde Integration</a> section.</p>
<h2 id="cursors"><a class="header" href="#cursors">Cursors</a></h2>
<p>Results from queries are generally returned via <a href="https://docs.rs/mongodb/latest/mongodb/struct.Cursor.html"><code>Cursor</code></a>, a struct which streams the results back from the server as requested. The <code>Cursor</code> type implements the <a href="https://docs.rs/futures/latest/futures/stream/trait.Stream.html"><code>Stream</code></a> trait from the <a href="https://crates.io/crates/futures"><code>futures</code></a> crate, and in order to access its streaming functionality you need to import at least one of the <a href="https://docs.rs/futures/latest/futures/stream/trait.StreamExt.html"><code>StreamExt</code></a> or <a href="https://docs.rs/futures/latest/futures/stream/trait.TryStreamExt.html"><code>TryStreamExt</code></a> traits.</p>
<pre><code class="language-toml"># In Cargo.toml, add the following dependency.
futures = "0.3"
</code></pre>
<pre><pre class="playground"><code class="language-rust no_run edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span><span class="boring">extern crate mongodb;
</span><span class="boring">extern crate serde;
</span><span class="boring">extern crate futures;
</span><span class="boring">use serde::Deserialize;
</span><span class="boring">#[derive(Deserialize)]
</span><span class="boring">struct Book { title: String }
</span><span class="boring">async fn foo() -> mongodb::error::Result<()> {
</span><span class="boring">let typed_collection = mongodb::Client::with_uri_str("").await?.database("").collection::<Book>("");
</span>// This trait is required to use `try_next()` on the cursor
use futures::stream::TryStreamExt;
use mongodb::{bson::doc, options::FindOptions};
// Query the books in the collection with a filter and an option.
let filter = doc! { "author": "George Orwell" };
let find_options = FindOptions::builder().sort(doc! { "title": 1 }).build();
let mut cursor = typed_collection.find(filter, find_options).await?;
// Iterate over the results of the cursor.
while let Some(book) = cursor.try_next().await? {
println!("title: {}", book.title);
}
<span class="boring">Ok(()) }
</span><span class="boring">}</span></code></pre></pre>
<p>If a <a href="https://docs.rs/mongodb/latest/mongodb/struct.Cursor.html"><code>Cursor</code></a> is still open when it goes out of scope, it will automatically be closed via an asynchronous <a href="https://www.mongodb.com/docs/manual/reference/command/killCursors/">killCursors</a> command executed from its <a href="https://doc.rust-lang.org/std/ops/trait.Drop.html"><code>Drop</code></a> implementation.</p>
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