Refactor DB code with Database(Holder|Helper) #245
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Including only what is used should reduce compilation time. The added includes were necessary to make the respective translation units compile after a thorough cleanup of data_base_management.h includes. This is how the complete include and forward-declaration sections should look like in data_base_management.h: '#'include <QThread> // (prevent '#' from starting a git comment line) class QSqlDatabase; class QSqlQuery; class QSqlRecord; class QString; class QStringList; Subsequently I had to partly restore the unnecessary includes by moving them from data_base_management.cpp into the header due to the huge number of compilation errors I am not prepared to deal with right now.
Even an implicitly shared class is cheaper to pass by reference than by value, as evidenced by Qt APIs themselves.
All other similar updating functions take db as the last argument.
Take QString arguments by value only if they are unconditionally moved from. Fix typos in DBHelper::reassignOrderTo* member function names.
These classes will allow to improve exception and early-return safety as well as simplify code that calls QSqlDatabase::removeDatabase().
The returned value is always the same and is not used by the caller. Remove unused variables from this function.
DatabaseHolder is preferable to DatabaseHelper here for these reasons: 1. DataBaseManagement uses DBHelper only once, so DatabaseHelper's advantage is tiny here. 2. DataBaseManagement uses the wrapped QSqlDatabase object a lot. DatabaseHolder's operators make these uses convenient and less verbose. 3. In YACReader code a QSqlDatabase object is almost always obtained via DataBaseManagement::loadDatabase(). DatabaseHelper provides only one constructor that adds a QSqlDatabase connection in this way to prevent usage mistakes. By contrast, DataBaseManagement always adds QSqlDatabase connections in other ways, so DatabaseHelper cannot be used as is.
This change eliminates the following warning from the output of YACReaderLibrary built in Debug mode: QSqlDatabasePrivate::addDatabase: duplicate connection name '/path/to/.yacreaderlibrary<thread id>', old connection removed.
This change shows how code is simplified by using DatabaseHelper. It also allows to test DatabaseHelper. Eventually DatabaseHelper (or rarely DatabaseHolder) should replace almost all explicit QSqlDatabase::removeDatabase() calls.
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