Fixed foreign-key (mariadb) errors.

When using MariaDB v10.5+ Foreign-Key errors were popping up because of
some changes in that version. To mitigate this on MariaDB and other
MySQL forks those errors are now catched, and instead of a replace_into
an update will happen. I have tested this as thorough as possible with
MariaDB 10.5, 10.4, 10.3 and the default MySQL on Ubuntu Focal. And
tested it again using sqlite, all seems to be ok on all tables.

resolves #1081. resolves #1065, resolves #1050

src/db/models/attachment.rs

@@ -63,10 +63,21 @@ impl Attachment {
     pub fn save(&self, conn: &DbConn) -> EmptyResult {
         db_run! { conn:
             sqlite, mysql {
-                diesel::replace_into(attachments::table)
+                match diesel::replace_into(attachments::table)
                     .values(AttachmentDb::to_db(self))
                     .execute(conn)
-                    .map_res("Error saving attachment")
+                {
+                    Ok(_) => Ok(()),
+                    // Record already exists and causes a Foreign Key Violation because replace_into() wants to delete the record first.
+                    Err(diesel::result::Error::DatabaseError(diesel::result::DatabaseErrorKind::ForeignKeyViolation, _)) => {
+                        diesel::update(attachments::table)
+                            .filter(attachments::id.eq(&self.id))
+                            .set(AttachmentDb::to_db(self))
+                            .execute(conn)
+                            .map_res("Error saving attachment")
+                    }
+                    Err(e) => Err(e.into()),
+                }.map_res("Error saving attachment")
             }
             postgresql {
                 let value = AttachmentDb::to_db(self);