Trait diesel::prelude::Connection

source · 
pub trait Connection:
    SimpleConnection
    + Sized
    + Send
where
    Self: ConnectionSealed,
{
    type Backend: Backend;

    // Required methods
    fn establish(database_url: &str) -> ConnectionResult<Self>;
    fn set_instrumentation(&mut self, instrumentation: impl Instrumentation);

    // Provided methods
    fn transaction<T, E, F>(&mut self, f: F) -> Result<T, E>
       where F: FnOnce(&mut Self) -> Result<T, E>,
             E: From<Error> { ... }
    fn begin_test_transaction(&mut self) -> QueryResult<()> { ... }
    fn test_transaction<T, E, F>(&mut self, f: F) -> T
       where F: FnOnce(&mut Self) -> Result<T, E>,
             E: Debug { ... }
}
Expand description

A connection to a database

This trait represents a database connection. It can be used to query the database through the query dsl provided by diesel, custom extensions or raw sql queries.

§Implementing a custom connection

There are several reasons why you would want to implement a custom connection implementation:

  • To wrap an existing connection for instrumentation purposes
  • To use a different underlying library to provide a connection implementation for already existing backends.
  • To add support for an unsupported database system

Implementing a Connection in a third party crate requires enabling the i-implement-a-third-party-backend-and-opt-into-breaking-changes crate feature which grants access to some of diesel’s implementation details.

§Wrapping an existing connection impl

Wrapping an existing connection allows you to customize the implementation to add additional functionality, like for example instrumentation. For this use case you only need to implement Connection, LoadConnection and all super traits. You should forward any method call to the wrapped connection type. It is important to also forward any method where diesel provides a default implementation, as the wrapped connection implementation may contain a customized implementation.

To allow the integration of your new connection type with other diesel features it may be useful to also implement R2D2Connection and MigrationConnection.

§Provide a new connection implementation for an existing backend

Implementing a new connection based on an existing backend can enable the usage of other methods to connect to the database. One example here would be to replace the official diesel provided connection implementations with an implementation based on a pure rust connection crate.

It’s important to use prepared statements to implement the following methods:

For performance reasons it may also be meaningful to cache already prepared statements. See StatementCache for a helper type to implement prepared statement caching. The statement_cache module documentation contains details about efficient prepared statement caching based on diesels query builder.

It is required to implement at least the following parts:

  • A row type that describes how to receive values form a database row. This type needs to implement Row
  • A field type that describes a database field value. This type needs to implement Field
  • A connection type that wraps the connection + the necessary state management.
  • Maybe a TransactionManager implementation matching the interface provided by the database connection crate. Otherwise the implementation used by the corresponding Connection in diesel can be reused.

To allow the integration of your new connection type with other diesel features it may be useful to also implement R2D2Connection and MigrationConnection.

The exact implementation of the Connection trait depends on the interface provided by the connection crate/library. A struct implementing Connection should likely contain a StatementCache to cache prepared statements efficiently.

As implementations differ significantly between the supported backends we cannot give a one for all description here. Generally it’s likely a good idea to follow the implementation of the corresponding connection in diesel at a high level to gain some idea how to implement your custom implementation.

§Implement support for an unsupported database system

Additionally to anything mentioned in the previous section the following steps are required:

  • Implement a custom backend type. See the documentation of Backend for details
  • Implement appropriate FromSql/ ToSql conversions. At least the following impls should be considered:
    • i16: FromSql<SmallInt, YourBackend>
    • i32: FromSql<Integer, YourBackend>
    • i64: FromSql<BigInt, YourBackend>
    • f32: FromSql<Float, YourBackend>
    • f64: FromSql<Double, YourBackend>
    • bool: FromSql<Bool, YourBackend>
    • String: FromSql<Text, YourBackend>
    • Vec<u8>: FromSql<Binary, YourBackend>
    • i16: ToSql<SmallInt, YourBackend>
    • i32: ToSql<Integer, YourBackend>
    • i64: ToSql<BigInt, YourBackend>
    • f32: ToSql<Float, YourBackend>
    • f64: ToSql<Double, YourBackend>
    • bool: ToSql<Bool, YourBackend>
    • String: ToSql<Text, YourBackend>
    • Vec<u8>: ToSql<Binary, YourBackend>
  • Maybe a TransactionManager implementation matching the interface provided by the database connection crate. Otherwise the implementation used by the corresponding Connection in diesel can be reused.

As these implementations will vary depending on the backend being used, we cannot give concrete examples here. We recommend looking at our existing implementations to see how you can implement your own connection.

Required Associated Types§

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type Backend: Backend

The backend this type connects to

Required Methods§

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fn establish(database_url: &str) -> ConnectionResult<Self>

Establishes a new connection to the database

The argument to this method and the method’s behavior varies by backend. See the documentation for that backend’s connection class for details about what it accepts and how it behaves.

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fn set_instrumentation(&mut self, instrumentation: impl Instrumentation)

Set a specific Instrumentation implementation for this connection

Provided Methods§

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fn transaction<T, E, F>(&mut self, f: F) -> Result<T, E>
where F: FnOnce(&mut Self) -> Result<T, E>, E: From<Error>,

Executes the given function inside of a database transaction

This function executes the provided closure f inside a database transaction. If there is already an open transaction for the current connection savepoints will be used instead. The connection is committed if the closure returns Ok(_), it will be rolled back if it returns Err(_). For both cases the original result value will be returned from this function.

If the transaction fails to commit due to a SerializationFailure or a ReadOnlyTransaction a rollback will be attempted. If the rollback fails, the error will be returned in a Error::RollbackErrorOnCommit, from which you will be able to extract both the original commit error and the rollback error. In addition, the connection will be considered broken as it contains a uncommitted unabortable open transaction. Any further interaction with the transaction system will result in an returned error in this case.

If the closure returns an Err(_) and the rollback fails the function will return that rollback error directly, and the transaction manager will be marked as broken as it contains a uncommitted unabortable open transaction.

If a nested transaction fails to release the corresponding savepoint the error will be returned directly.

§Example
use diesel::result::Error;

conn.transaction::<_, Error, _>(|conn| {
    diesel::insert_into(users)
        .values(name.eq("Ruby"))
        .execute(conn)?;

    let all_names = users.select(name).load::<String>(conn)?;
    assert_eq!(vec!["Sean", "Tess", "Ruby"], all_names);

    Ok(())
})?;

conn.transaction::<(), _, _>(|conn| {
    diesel::insert_into(users)
        .values(name.eq("Pascal"))
        .execute(conn)?;

    let all_names = users.select(name).load::<String>(conn)?;
    assert_eq!(vec!["Sean", "Tess", "Ruby", "Pascal"], all_names);

    // If we want to roll back the transaction, but don't have an
    // actual error to return, we can return `RollbackTransaction`.
    Err(Error::RollbackTransaction)
});

let all_names = users.select(name).load::<String>(conn)?;
assert_eq!(vec!["Sean", "Tess", "Ruby"], all_names);
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fn begin_test_transaction(&mut self) -> QueryResult<()>

Creates a transaction that will never be committed. This is useful for tests. Panics if called while inside of a transaction or if called with a connection containing a broken transaction

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fn test_transaction<T, E, F>(&mut self, f: F) -> T
where F: FnOnce(&mut Self) -> Result<T, E>, E: Debug,

Executes the given function inside a transaction, but does not commit it. Panics if the given function returns an error.

§Example
use diesel::result::Error;

conn.test_transaction::<_, Error, _>(|conn| {
    diesel::insert_into(users)
        .values(name.eq("Ruby"))
        .execute(conn)?;

    let all_names = users.select(name).load::<String>(conn)?;
    assert_eq!(vec!["Sean", "Tess", "Ruby"], all_names);

    Ok(())
});

// Even though we returned `Ok`, the transaction wasn't committed.
let all_names = users.select(name).load::<String>(conn)?;
assert_eq!(vec!["Sean", "Tess"], all_names);

Object Safety§

This trait is not object safe.

Implementors§