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