add doc for mod db

src/db.rs

@@ -3,16 +3,19 @@ use derive_more::{Display, Error};
 use log::info;
 use sqlx::postgres::PgPoolOptions;
 use sqlx::{Pool, Postgres};
-use std::io;
 
+/// `AppDB` represents for the database in use.
 #[derive(Clone)]
 pub enum AppDB {
-    Postgres(Pool<Postgres>),
+    /// Connect to a postgresql database.
+    Postgres(Pool<Postgres>), // Or any other database you use.
+    /// Manually return data.
     Mock,
 }
 
 impl AppDB {
-    pub async fn postgres(config: &AppConfig) -> io::Result<Self> {
+    /// Generates a database connection to postgresql
+    pub async fn postgres(config: &AppConfig) -> std::io::Result<Self> {
         let db_url = format!(
             "postgres://{}:{}@{}/{}",
             config.db_user, config.db_password, config.db_host, config.db_database
@@ -31,15 +34,21 @@ impl AppDB {
             Ok(Self::Postgres(pool))
         } else {
             let msg = format!("failed to connect to database, config {:?}", config);
-            Err(io::Error::other(msg))
+            Err(std::io::Error::other(msg))
         }
     }
+
+    /// Does not connect to any database.
+    ///
+    /// Any data is generated by hand.
     pub fn mock() -> Self {
         Self::Mock
     }
 }
 
+// Example implementation to a database operation.
 impl AppDB {
+    /// Tests whether the connection is successful.
     pub async fn test_db(&self) -> DBResult<()> {
         match self {
             Self::Postgres(pool) => {
@@ -51,13 +60,16 @@ impl AppDB {
                 assert_eq!(row.0, 150);
                 Ok(())
             }
+            // Does not implement for `Mock`
             _ => Err(DBError::Unimplemented),
         }
     }
 }
 
+/// `DBResult` is used in database operation methods.
 pub type DBResult<T> = std::result::Result<T, DBError>;
 
+/// `DBError` represents for all the possible errors that may occur in a database operation.
 #[derive(Debug, Error, Display)]
 pub enum DBError {
     SqlxError(sqlx::Error),