When using QueryBuilder, why is the result of the build() functions bound to its builder?

[nerdo]

Here's a concrete example of what I'm trying to do and why I'm trying to understand this:

// I'm not sure how to do something like this properly. Basically, I need to have some functions
// that build queries that will get executed from another function. There is an ownership/borrowing
// issue that I can't seem to resolve. What's the right way to do something like this?

fn build_issues_query_count(_settings: &ChiliQuerySettings) -> MySqlQuery {
    let mut query = sqlx::QueryBuilder::new("SELECT COUNT(1) from `issues`");

    // Refine query to filter data here...

    query.build()
    //         error[E0515]: cannot return reference to local variable `query`
    //   --> src/main.rs:10:5
    //    |
    // 10 |     query.build()
    //    |     ^^^^^^^^^^^^^ returns a reference to data owned by the current function
}

fn build_issues_query(_settings: &ChiliQuerySettings) -> IssuesQuery {
    let mut query = sqlx::QueryBuilder::new(
        r#"SELECT 
                 `id`, 
                 `tracker_id`,
                 `subject`,
                 `description`,
            FROM `issues`
        "#,
    );

    // Refine query to filter data here...

    query.build_query_as::<Issue>()
    // error[E0515]: cannot return reference to local variable `query`
    //   --> src/main.rs:31:5
    //    |
    // 31 |     query.build_query_as::<Issue>()
    //    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ returns a reference to data owned by the current function
}

/// Structs
pub struct Issue {
    /// Issue ID.
    pub id: i32,

    /// Tracker ID.
    pub tracker_id: i32,

    /// Subject of issue.
    pub subject: String,

    /// Description of issue.
    pub description: Option<String>,
}
type MySqlQuery = sqlx::query::Query<'static, sqlx::MySql, sqlx::mysql::MySqlArguments>;
type IssuesQuery = sqlx::query::QueryAs<'static, sqlx::MySql, Issue, sqlx::mysql::MySqlArguments>;

/// Settings for how to query the Chili database.
#[derive(Clone, Debug)]
pub struct ChiliQuerySettings {
    /// Where clause to apply to the issues query.
    /// Can be used to limit the records to a specific project, for example.
    pub issues_query_tracker_ids: Option<Vec<i32>>,
}

impl sqlx::FromRow<'_, sqlx::mysql::MySqlRow> for Issue {
    fn from_row(_row: &'_ sqlx::mysql::MySqlRow) -> Result<Self, sqlx::Error> {
        todo!()
    }
}

// Cargo.toml
// [package]
// name = "sqlx_query_building_example"
// version = "0.1.0"
// edition = "2021"

// # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

// [dependencies]
// sqlx = { version = "0.7.1", features = ["mysql"] }

I eventually got some help in a Rust discord and was told I could use Box::leak() to effectively turn the lifetime of query into 'static as I understand it - leak the memory so Rust doesn't try to clean it up at the end of the function.

It works, but it doesn't feel right. It feels very hacky. But I think I understand it. Since query is owned by the function and build doesn't seem to take ownership of it, I don't know any other way around it.

The main question I have is why are the resulting Query and the QueryBuilder tied together in this way in the first place? It feels like the builder should build and if it needs any data to run the query, it should have been, well, built (or cloned) at that point.

Or am I just misunderstanding how this all works and doing it all wrong? Please help me understand.

[jrudolph]

I have the same question. Note, that it works for static query strings in query_as which just borrows the query string and bind its own lifetime to that of the result.

For QueryBuilder it seems to make even less sense since it owns the query. If the query builder would pass ownership of itself to the result, it seems the problem could be solved.

In neither case, you can return the result stream if you need a dynamic query string.