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diff --git a/Cargo.toml b/Cargo.toml
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -12,3 +12,4 @@
log = "^0.4.4"
r2d2 = "^0.8.2"
rocket = "^0.3.16"
+rocket_contrib = { version = "*", features = ["json"] }
diff --git a/src/api/mod.rs b/src/api/mod.rs
new file mode 100644
--- /dev/null
+++ b/src/api/mod.rs
@@ -0,0 +1,9 @@
+//! # API module
+//!
+//! This module provides useful code to create easily APIs.
+
+/* -------------------------------------------------------------
+ Public submodules offered by this module
+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
+
+pub mod replies;
diff --git a/src/api/replies.rs b/src/api/replies.rs
new file mode 100644
--- /dev/null
+++ b/src/api/replies.rs
@@ -0,0 +1,128 @@
+//! # API module
+//!
+//! This module provides useful traits and methods to craft API replies from an existing type.
+
+use std::error::Error;
+
+use diesel::result::DatabaseErrorInformation;
+use diesel::result::DatabaseErrorKind;
+use diesel::result::Error as ResultError;
+use diesel::result::QueryResult;
+use rocket::http::Status;
+use rocket::response::Failure;
+use rocket_contrib::Json;
+
+/* -------------------------------------------------------------
+ Custom types
+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
+
+pub type ApiJsonResponse<T> = Result<Json<T>, Failure>;
+
+/* -------------------------------------------------------------
+ API Response
+
+ :: Implementation for QueryResult (Diesel ORM)
+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
+
+/// This trait allows to consume an object into an HTTP response.
+pub trait ApiResponse<T> {
+ /// Consumes the value and creates a JSON or a Failure result response.
+ fn into_json_response(self) -> ApiJsonResponse<T>;
+}
+
+impl<T> ApiResponse<T> for QueryResult<T> {
+ /// Prepares an API response from a query result.
+ ///
+ /// The result is the data structure prepared by the Diesel ORM after a SELECT query
+ /// with one result, for example using `first` method. You can also you use it to
+ /// parse the returning result (... RETURNING *), which is a default for Diesel after
+ /// an INSERT query.
+ ///
+ /// So result can be:
+ /// - Ok(T)
+ /// - Err(any database error)
+ ///
+ /// # Examples
+ ///
+ /// To offer a /player/foo route to serve player information from the player table:
+ ///
+ /// ```
+ /// use limiting_factor::api::ApiResponse;
+ /// use limiting_factor::api::ApiJsonResponse;
+ ///
+ /// #[get("/player/<name>")]
+ /// pub fn get_player(connection: DatabaseConnection, name: String) -> ApiJsonResponse<Player> {
+ /// players
+ /// .filter(username.eq(&name))
+ /// .first::<Player>(&*connection)
+ /// .into_json_response()
+ /// }
+ /// ```
+ ///
+ /// This will produce a JSON representation when the result is found,
+ /// a 404 error when no result is found, a 500 error if there is a database issue.
+ fn into_json_response(self) -> ApiJsonResponse<T> {
+ match self {
+ // CASE I - The query returns one value, we return a JSON representation fo the item
+ Ok(item) => Ok(Json(item)),
+
+ Err(error) => match error {
+ // Case II - The query returns no result, we return a 404 Not found response
+ ResultError::NotFound => Err(Failure::from(Status::NotFound)),
+
+ // Case III - We need to handle a database error, which could be a 400/409/500
+ ResultError::DatabaseError(kind, details) => Err(build_database_error_response(kind, details)),
+
+ // Case IV - The error is probably server responsbility, log it and throw a 500
+ _ => Err(error.into_failure_response()),
+ }
+ }
+ }
+}
+
+/* -------------------------------------------------------------
+ Failure response
+
+ :: Implementation for diesel::result::Error
+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
+
+/// This trait allows to consume an object into an HTTP failure response.
+pub trait FailureResponse {
+ /// Consumes the variable and creates a Failure response .
+ fn into_failure_response(self) -> Failure;
+}
+
+impl FailureResponse for ResultError {
+ /// Consumes the error and creates a Failure 500 Internal server error response.
+ fn into_failure_response(self) -> Failure {
+ build_internal_server_error_response(self.description())
+ }
+}
+
+/* -------------------------------------------------------------
+ Helper methods to prepare API responses
+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
+
+pub fn build_internal_server_error_response(message: &str) -> Failure {
+ warn!(target:"api", "{}", message);
+
+ Failure::from(Status::InternalServerError)
+}
+
+fn build_database_error_response(error_kind: DatabaseErrorKind, info: Box<dyn DatabaseErrorInformation>) -> Failure {
+ match error_kind {
+ // Case IIIa - The query tries to do an INSERT violating an unique constraint
+ // e.g. two INSERT with the same unique value
+ // We return a 409 Conflict
+ DatabaseErrorKind::UniqueViolation => Failure::from(Status::Conflict),
+
+ // Case IIIb - The query violated a foreign key constraint
+ // e.g. an INSERT referring to a non existing user 1004
+ // when there is no id 1004 in users table
+ // We return a 400 Bad request
+ DatabaseErrorKind::ForeignKeyViolation => Failure::from(Status::BadRequest),
+
+ // Case IIIc - For other databases errors, the client responsibility isn't involved.
+ _ => build_internal_server_error_response(info.message()),
+ }
+}
diff --git a/src/lib.rs b/src/lib.rs
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -2,11 +2,13 @@
#[macro_use] extern crate log;
extern crate r2d2;
extern crate rocket;
+extern crate rocket_contrib;
/* -------------------------------------------------------------
Public modules offered by this crate
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
+pub mod api;
pub mod database;
/* -------------------------------------------------------------

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