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void/Omnivore/2023-10-25 - What the Rust Book didn't tell you about testing....md

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+---
+id: ad8a0732-733a-11ee-b2ab-13f1d6012322
+title: |
+  What the Rust Book didn't tell you about testing...
+status: ARCHIVED
+tags:
+  - read-later
+  - Newsletter
+date_added: 2023-10-25 10:30:35
+url_omnivore: |
+  https://omnivore.app/me/what-the-rust-book-didn-t-tell-you-about-testing-18b6707a120
+url_original: |
+  https://omnivore.app/no_url?q=1fed5fd7-7706-46cf-9947-927b26a77112
+---
+
+# What the Rust Book didn't tell you about testing...
+
+## Highlights
+
+We want to test _get\_user_ without making real database queries.
+
+The solution is to mock the _Database_ trait and assert _execute\_query_ is called with the correct query. But how?
+
+We can use the [mockall crate](https://letsgetrusty.us6.list-manage.com/track/click?u=9f28b35c1658c447f3b962a54&id=00a07042b3&e=d0eb971086)! 
+
+[source](https://omnivore.app/me/what-the-rust-book-didn-t-tell-you-about-testing-18b6707a120#f9650419-c778-4974-9da2-aabce209609f) 
+
+---
+
+## Original
+
+  
+The Rust Book has an [entire chapter dedicated to testing](https://letsgetrusty.us6.list-manage.com/track/click?u=9f28b35c1658c447f3b962a54&id=a95a715235&e=d0eb971086), but it's missing a critical piece…
+
+What happens when your code needs to make API calls or database queries?
+
+Unit tests should be be fast, reliable, and deterministic.
+
+We don't want to make expensive calls that might fail for various reasons.
+
+  
+Well here is some good news… we don't have to!
+
+We can use mocking to substitute real objects for mock objects and assert certain expectations…
+
+What's that? You want an example? Consider the following code…
+
+trait Database {
+    fn execute_query(&self, query: String);
+}
+
+fn get_user(db: impl Database, id: i32) {
+    let query = format!("SELECT * from Users where id={}", id);
+    db.execute_query(query);
+}
+We want to test _get\_user_ without making real database queries.
+
+The solution is to mock the _Database_ trait and assert _execute\_query_ is called with the correct query. But how?
+
+We can use the [mockall crate](https://letsgetrusty.us6.list-manage.com/track/click?u=9f28b35c1658c447f3b962a54&id=00a07042b3&e=d0eb971086)!
+
+Here is how we would test _get\_user_…
+
+#[cfg(test)]
+use mockall::{automock, predicate::*};
+
+#[cfg_attr(test, automock)]
+trait Database {
+    fn execute_query(&self, query: String);
+}
+
+fn get_user(db: impl Database, id: i32) {
+    let query = format!("SELECT * from Users where id={}", id);
+    db.execute_query(query);
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn get_user_executes_correct_query() {
+        let mut mock_database = MockDatabase::new();
+        mock_database.expect_execute_query()
+            .with(eq("SELECT * from Users where id=22".to_owned()))
+            .once()
+            .returning(|_x| ());
+
+        get_user(mock_database, 22);
+    }
+}
+Boom! Now we have a unit test that's fast, reliable, and deterministic!
+
+If you haven't seen my [intro to testing in Rust video](https://letsgetrusty.us6.list-manage.com/track/click?u=9f28b35c1658c447f3b962a54&id=90d4167901&e=d0eb971086) make sure to check it out!  
+  
+  
+Happy testing, and stay Rusty!