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POPersistence classes in Qt
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copied!<p>I'm porting a medium-sized CRUD application from .Net to Qt and I'm looking for a pattern for creating persistence classes. In .Net I usually created abstract persistence class with basic methods (insert, update, delete, select) for example:</p> <pre><code>public class DAOBase<T> { public T GetByPrimaryKey(object primaryKey) {...} public void DeleteByPrimaryKey(object primaryKey) {...} public List<T> GetByField(string fieldName, object value) {...} public void Insert(T dto) {...} public void Update(T dto) {...} } </code></pre> <p>Then, I subclassed it for specific tables/DTOs and added attributes for DB table layout:</p> <pre><code>[DBTable("note", "note_id", NpgsqlTypes.NpgsqlDbType.Integer)] [DbField("note_id", NpgsqlTypes.NpgsqlDbType.Integer, "NoteId")] [DbField("client_id", NpgsqlTypes.NpgsqlDbType.Integer, "ClientId")] [DbField("title", NpgsqlTypes.NpgsqlDbType.Text, "Title", "")] [DbField("body", NpgsqlTypes.NpgsqlDbType.Text, "Body", "")] [DbField("date_added", NpgsqlTypes.NpgsqlDbType.Date, "DateAdded")] class NoteDAO : DAOBase<NoteDTO> { } </code></pre> <p>Thanks to .Net reflection system I was able to achieve heavy code reuse and easy creation of new ORMs.</p> <p>The simplest way to do this kind of stuff in Qt seems to be using model classes from QtSql module. Unfortunately, in my case they provide too abstract an interface. I need at least transactions support and control over individual commits which QSqlTableModel doesn't provide.</p> <p>Could you give me some hints about solving this problem using Qt or point me to some reference materials?</p> <hr> <p><strong>Update:</strong></p> <p>Based on Harald's clues I've implemented a solution that is quite similar to the .Net classes above. Now I have two classes.</p> <p><em>UniversalDAO</em> that inherits <em>QObject</em> and deals with <em>QObject</em> DTOs using metatype system:</p> <pre><code>class UniversalDAO : public QObject { Q_OBJECT public: UniversalDAO(QSqlDatabase dataBase, QObject *parent = 0); virtual ~UniversalDAO(); void insert(const QObject &dto); void update(const QObject &dto); void remove(const QObject &dto); void getByPrimaryKey(QObject &dto, const QVariant &key); }; </code></pre> <p>And a generic <em>SpecializedDAO</em> that casts data obtained from <em>UniversalDAO</em> to appropriate type:</p> <pre><code>template<class DTO> class SpecializedDAO { public: SpecializedDAO(UniversalDAO *universalDao) virtual ~SpecializedDAO() {} DTO defaultDto() const { return DTO; } void insert(DTO dto) { dao->insert(dto); } void update(DTO dto) { dao->update(dto); } void remove(DTO dto) { dao->remove(dto); } DTO getByPrimaryKey(const QVariant &key); }; </code></pre> <p>Using the above, I declare the concrete DAO class as following:</p> <pre><code>class ClientDAO : public QObject, public SpecializedDAO<ClientDTO> { Q_OBJECT public: ClientDAO(UniversalDAO *dao, QObject *parent = 0) : QObject(parent), SpecializedDAO<ClientDTO>(dao) {} }; </code></pre> <p>From within <em>ClientDAO</em> I have to set some database information for <em>UniversalDAO</em>. That's where my implementation gets ugly because I do it like this:</p> <pre><code>QMap<QString, QString> fieldMapper; fieldMapper["client_id"] = "clientId"; fieldMapper["name"] = "firstName"; /* ...all column <-> field pairs in here... */ dao->setFieldMapper(fieldMapper); dao->setTable("client"); dao->setPrimaryKey("client_id"); </code></pre> <p>I do it in constructor so it's not visible at a first glance for someone browsing through the header. In .Net version it was easy to spot and understand.</p> <p>Do you have some ideas how I could make it better?</p>

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