How to use custom type for sqlx::query_as result? Getting "trait bound FromRow is not satisfied" error

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04-10-2024

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Demystifying the "trait bound FromRow is not satisfied" Error in SQLx: Custom Types and Query Results

Have you encountered the frustrating "trait bound FromRow is not satisfied" error while using SQLx to query your database with custom types? This error often arises when you're trying to map the results of your SQL query to a custom data structure, and SQLx can't figure out how to automatically convert the data into your type.

Let's break down this common issue and equip you with the knowledge to gracefully handle custom types in your SQLx queries.

The Scenario:

Imagine you have a table called "users" with columns like "id", "username", and "email". You want to fetch user data and represent it using a Rust struct:

#[derive(Debug, Clone)]
struct User {
    id: i32,
    username: String,
    email: String,
}

Your SQL query looks like this:

let sql = "SELECT id, username, email FROM users";
let users = sqlx::query_as::<_, User>(sql).fetch_all(&pool).await?;

But, instead of returning a list of User structs, you're greeted with the dreaded error: "trait bound FromRow is not satisfied".

Understanding the Problem:

The sqlx::query_as function relies on the FromRow trait to map database rows to your desired Rust type. However, SQLx can only automatically derive FromRow for primitive types and types that implement serde::Deserialize. Your User struct, while perfectly valid, doesn't fall into either of these categories.

The Solution:

To make your custom User struct work with sqlx::query_as, you need to explicitly implement the FromRow trait for it:

#[derive(Debug, Clone)]
struct User {
    id: i32,
    username: String,
    email: String,
}

#[sqlx::FromRow]
impl<'r> User for sqlx::Row<'r> {
    fn from_row(row: &sqlx::Row<'r>) -> Result<User, sqlx::Error> {
        Ok(User {
            id: row.get(0)?,
            username: row.get(1)?,
            email: row.get(2)?,
        })
    }
}

By implementing FromRow, you're telling SQLx how to extract data from a database row and construct a User instance. Here's a breakdown:

1. #[sqlx::FromRow]: This macro informs SQLx that you're providing a custom FromRow implementation.
2. impl<'r> User for sqlx::Row<'r>: The implementation takes a reference to a sqlx::Row (which represents a single row from your query).
3. fn from_row(row: &sqlx::Row<'r>) -> Result<User, sqlx::Error>: The function takes the sqlx::Row and extracts the relevant data using row.get(index).
4. Ok(User {...}): Finally, it returns a Result containing a User struct constructed with the extracted values.

Additional Considerations:

• Column Order: Ensure that the order of fields in your User struct matches the order of columns in your SQL query.
• Data Types: Double-check that the data types in your SQL query (e.g., i32 for id) match the corresponding types in your User struct.
• Complex Data Structures: For more complex scenarios with nested data structures, consider leveraging libraries like serde to streamline the deserialization process.

Conclusion:

By understanding the FromRow trait and implementing it for your custom types, you gain complete control over how SQLx maps database rows to your desired Rust structures. Remember to always pay attention to data types and column order for seamless data extraction.

References:

• SQLx Documentation
• Serde Documentation