Simplifying Data Access with Database Attributes
This topic demonstrates the use of database attributes to simplify database operations.
The basic way to access information from a database is to create a command (or table) class and a user record class for a particular table in the database. The database attributes simplify some of the template declarations that you previously had to do.
To demonstrate the use of database attributes, the following sections show two equivalent table and user record class declarations: the first uses attributes and the second uses OLE DB Templates. Such declaration code is typically placed in a header file named for the table or command object, for example, Authors.h.
By comparing the two files, you can see how much simpler it is to use attributes. Among the differences are:
Using attributes, you only have to declare one class: CAuthors, while with templates you have to declare two: CAuthorsNoAttrAccessor and CAuthorsNoAttr.
The db_source call in the attributed version is equivalent to the OpenDataSource() call in the template declaration.
The db_table call in the attributed version is equivalent to the following template declaration:
class CAuthorsNoAttr : public CTable<CAccessor<CAuthorsNoAttrAccessor> >The db_column calls in the attributed version are equivalent to the column map (see BEGIN_COLUMN_MAP ... END_COLUMN_MAP) in the template declaration.
The attributes inject a user record class declaration for you. The user record class is equivalent to CAuthorsNoAttrAccessor in the template declaration. If your table class is CAuthors, the injected user record class is named CAuthorsAccessor, and you can only view its declaration in injected code. For more information, see "Attribute-Injected User Record Classes" in User Records.
Note that in both the attributed and the templated code, you must set rowset properties using CDBPropSet::AddProperty.
For information about the attributes discussed in this topic, see OLE DB Consumer Attributes.
Table and Accessor Declaration Using Attributes
The following code calls db_source and db_table on the table class. db_source specifies the data source and connection to be used. db_table injects the appropriate template code to declare a table class. db_column specify the column map and inject the accessor declaration. You can use OLE DB consumer attributes in any project that supports ATL.
Here is the table and accessor declaration using attributes:
//////////////////////////////////////////////////////////////////////
// Table and accessor declaration using attributes
// authors.h
//////////////////////////////////////////////////////////////////////
// Table class declaration
// (Note that you must provide your own connection string for db_source.)
[
db_source(L"your connection string"),
db_table("Authors")
]
class CAuthors
{
public:
DWORD m_dwAuIDStatus;
DWORD m_dwAuthorStatus;
DWORD m_dwYearBornStatus;
DWORD m_dwAuIDLength;
DWORD m_dwAuthorLength;
DWORD m_dwYearBornLength;
[ db_column(1, status=m_dwAuIDStatus, length=m_dwAuIDLength) ] LONG m_AuID;
[ db_column(2, status=m_dwAuthorStatus, length=m_dwAuthorLength) ] TCHAR m_Author[51];
[ db_column(3, status=m_dwYearBornStatus, length=m_dwYearBornLength) ] SHORT m_YearBorn;
void GetRowsetProperties(CDBPropSet* pPropSet)
{
pPropSet->AddProperty(DBPROP_CANFETCHBACKWARDS, true);
pPropSet->AddProperty(DBPROP_CANSCROLLBACKWARDS, true);
pPropSet->AddProperty(DBPROP_IRowsetChange, true);
}
};
Table and Accessor Declaration Using Templates
Here is the table and accessor declaration using templates.
//////////////////////////////////////////////////////////////////////
// Table and user record class declaration using templates
// authors.h
//////////////////////////////////////////////////////////////////////
// User record class declaration
class CAuthorsNoAttrAccessor
{
public:
DWORD m_dwAuIDStatus;
DWORD m_dwAuthorStatus;
DWORD m_dwYearBornStatus;
DWORD m_dwAuIDLength;
DWORD m_dwAuthorLength;
DWORD m_dwYearBornLength;
LONG m_AuID;
TCHAR m_Author[51];
SHORT m_YearBorn;
void GetRowsetProperties(CDBPropSet* pPropSet)
{
pPropSet->AddProperty(DBPROP_CANFETCHBACKWARDS, true);
pPropSet->AddProperty(DBPROP_CANSCROLLBACKWARDS, true);
pPropSet->AddProperty(DBPROP_IRowsetChange, true);
}
HRESULT OpenDataSource()
{
CDataSource _db;
HRESULT hr;
hr = _db.OpenFromInitializationString(L"your connection string");
if (FAILED(hr))
{
#ifdef _DEBUG
AtlTraceErrorRecords(hr);
#endif
return hr;
}
return m_session.Open(_db);
}
void CloseDataSource()
{
m_session.Close();
}
operator const CSession&()
{
return m_session;
}
CSession m_session;
BEGIN_COLUMN_MAP(CAuthorsNoAttrAccessor)
COLUMN_ENTRY_LENGTH_STATUS(1, m_AuID, m_dwAuIDLength, m_dwAuIDStatus)
COLUMN_ENTRY_LENGTH_STATUS(2, m_Author, m_dwAuthorLength, m_dwAuthorStatus)
COLUMN_ENTRY_LENGTH_STATUS(3, m_YearBorn, m_dwYearBornLength, m_dwYearBornStatus)
END_COLUMN_MAP()
};
class CAuthorsNoAttr : public CTable<CAccessor<CAuthorsNoAttrAccessor> >
{
public:
HRESULT OpenAll()
{
HRESULT hr;
hr = OpenDataSource();
if (FAILED(hr))
return hr;
__if_exists(GetRowsetProperties)
{
CDBPropSet propset(DBPROPSET_ROWSET);
__if_exists(HasBookmark)
{
propset.AddProperty(DBPROP_IRowsetLocate, true);
}
GetRowsetProperties(&propset);
return OpenRowset(&propset);
}
__if_not_exists(GetRowsetProperties)
{
__if_exists(HasBookmark)
{
CDBPropSet propset(DBPROPSET_ROWSET);
propset.AddProperty(DBPROP_IRowsetLocate, true);
return OpenRowset(&propset);
}
}
return OpenRowset();
}
HRESULT OpenRowset(DBPROPSET *pPropSet = NULL)
{
HRESULT hr = Open(m_session, "Authors", pPropSet);
#ifdef _DEBUG
if(FAILED(hr))
AtlTraceErrorRecords(hr);
#endif
return hr;
}
void CloseAll()
{
Close();
CloseDataSource();
}
};