Fixing issue #759 by disabling Foreign Key Checks.

During migrations some queries are out of order regarding to foreign
keys.
Because of this the migrations fail when the sql database has this
enforced by default.
Turning of this check during the migrations will fix this and this is
only per session.

src/main.rs

@@ -323,6 +323,16 @@ mod migrations {
         let connection = crate::db::get_connection().expect("Can't connect to DB");
 
         use std::io::stdout;
+
+        // Disable Foreign Key Checks during migration
+        use diesel::RunQueryDsl;
+        #[cfg(feature = "postgres")]
+        diesel::sql_query("SET CONSTRAINTS ALL DEFERRED").execute(&connection).expect("Failed to disable Foreign Key Checks during migrations");
+        #[cfg(feature = "mysql")]
+        diesel::sql_query("SET FOREIGN_KEY_CHECKS = 0").execute(&connection).expect("Failed to disable Foreign Key Checks during migrations");
+        #[cfg(feature = "sqlite")]
+        diesel::sql_query("PRAGMA defer_foreign_keys = ON").execute(&connection).expect("Failed to disable Foreign Key Checks during migrations");
+
         embedded_migrations::run_with_output(&connection, &mut stdout()).expect("Can't run migrations");
     }
 }