#[automock] added only in test config but mock structure not recognized in integration test

ONLINECAST

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·

05-10-2024

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The Mocking Mystery: #[automock] in Test Config vs. Integration Tests

Problem: You've diligently added the #[automock] attribute to your code, hoping to simplify mocking in your tests. However, while the mocking works perfectly in your unit tests (configured in the test configuration), it inexplicably fails in your integration tests (configured in the integration_test configuration). You're baffled, staring at your code, wondering why the mocking magic isn't working in your integration environment.

Scenario:

Let's say you have a service that relies on a repository to fetch data:

pub struct MyService {
    repo: Box<dyn MyRepository>,
}

pub trait MyRepository {
    fn get_data(&self) -> String;
}

#[cfg(test)]
mod tests {
    use super::*;
    use mockall::automock;

    #[automock]
    trait MockMyRepository: MyRepository {}

    #[test]
    fn test_service_get_data() {
        let mock_repo = MockMyRepository::new();
        mock_repo.expect_get_data().returning(|| "Test Data".to_string());

        let service = MyService { repo: Box::new(mock_repo) };
        assert_eq!(service.get_data(), "Test Data");
    }
}

This code sets up a mock repository using mockall and works perfectly in unit tests. However, when you try to use the same mock in an integration test, it fails to recognize the MockMyRepository structure.

Analysis & Clarification:

The issue stems from the way #[automock] interacts with Rust's cargo configurations. By default, #[automock] only generates the mock struct in the specified test configuration, often test. This means that MockMyRepository is only available within the test configuration and not accessible in other configurations like integration_test.

Solution:

To resolve this, you have a few options:

1. Use #[automock] in the integration_test Configuration: The simplest solution is to add #[automock] within the integration_test configuration file, ensuring the mock structure is generated for both configurations. However, this might lead to unnecessary code generation if you only need the mock for unit tests.

2. Explicitly Generate the Mock Struct: You can manually generate the mock structure using mockall_derive::automock in the integration_test configuration:

   #[cfg(test)]
   #[cfg(feature = "integration_test")]
   mod integration_tests {
       use super::*;
       use mockall_derive::automock;
   
       #[automock]
       trait MockMyRepository: MyRepository {}
   
       // ... Your integration tests ...
   }
   

3. Move the Mock Trait to a Separate Module: Consider moving the trait definition and #[automock] to a separate module, and then bring it into both test and integration_test configurations using pub use. This allows for code reuse while ensuring the mock structure is available in both environments.

Example:

// src/my_service.rs
pub mod my_repository {
    pub trait MyRepository {
        fn get_data(&self) -> String;
    }

    #[cfg(test)]
    #[automock]
    pub trait MockMyRepository: MyRepository {}
}

pub struct MyService {
    repo: Box<dyn my_repository::MyRepository>,
}

pub fn get_data(&self) -> String {
    self.repo.get_data()
}

// tests/integration_tests.rs
#[cfg(test)]
#[cfg(feature = "integration_test")]
mod integration_tests {
    use super::*;
    use super::my_repository::MockMyRepository;

    // ... Your integration tests ...
}

Conclusion:

By understanding the behavior of #[automock] and considering the specific needs of your test configurations, you can avoid the common pitfall of mocking failures in integration tests. Choose the solution that best suits your project's structure and maintainability to ensure smooth and efficient testing across various environments.