Public Types | Static Public Member Functions

Wt::Dbo::dbo_traits< C > Class Template Reference
[Database Objects (Dbo)]

Traits for a class mapped with Wt::Dbo. More...

#include <Wt/Dbo/Dbo>

Inheritance diagram for Wt::Dbo::dbo_traits< C >:
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List of all members.

Public Types

typedef YourIdType IdType
 Type of the primary key.

Static Public Member Functions

static IdType invalidId ()
 Returns the sentinel value for a null id.
static const char * surrogateIdField ()
 Configures the surrogate primary key field.
static const char * versionField ()
 Configures the optimistic concurrency version field.

Detailed Description

template<class C>
class Wt::Dbo::dbo_traits< C >

Traits for a class mapped with Wt::Dbo.

The traits class provides some of the mapping properties related to the primary key and optimistic concurrency locking using a version field.

See dbo_default_traits for default values.

The following example changes the surrogate id field name for a class Foo from the default "id" to "foo_id":

 namespace Wt {
   namespace Dbo {

     template<>
     struct dbo_traits<Foo>
     {
        static const char *surrogateIdField() { return "foo_id"; }
     };
   }
 }

Member Typedef Documentation

template<class C>
typedef YourIdType Wt::Dbo::dbo_traits< C >::IdType

Type of the primary key.

This indicates the type of the primary key, which needs to be long long for a surrogate id, but can be any type supported by Wt::Dbo::field() (including composite types) for a natural primary key.

The following operations need to be supported for an id value:

  • default constructor
  • copy constructor
  • serialization to a string (for formatting an error message in exceptions) : std::ostream << id
  • comparison operator (for use as a key in a std::map): id == id
  • less than operator (for use as a key in a std::map): id < id

Only the default long long is supported for an auto-incrementing surrogate primary key. You need to change the default key type typically in conjuction with specifying a natural id, see Wt::Dbo::id().

The following example illustrates how to prepare a type to be usable as a composite id type:

 struct Coordinate {
   int x, y;

   Coordinate()
     : x(-1), y(-1) { }

   bool operator== (const Coordinate& other) const {
     return x == other.x && y == other.y;
   }

   bool operator< (const Coordinate& other) const {
     if (x < other.x)
       return true;
     else if (x == other.x)
       return y < other.y;
     else
       return false;
   }
 };
 
 std::ostream& operator<< (std::ostream& o, const Coordinate& c)
 {
   return o << "(" << c.x << ", " << c.y << ")";
 }
 
 namespace Wt {
   namespace Dbo {
 
     template <class Action>
     void field(Action& action, Coordinate& coordinate, const std::string& name, int size = -1)
     {
       field(action, coordinate.x, name + "_x");
       field(action, coordinate.y, name + "_y");
     }
   }
 }

Reimplemented from Wt::Dbo::dbo_default_traits.


Member Function Documentation

template<class C>
static IdType Wt::Dbo::dbo_traits< C >::invalidId (  ) [static]

Returns the sentinel value for a null id.

When used as a foreign key, this value is used to represent a null value.

Reimplemented from Wt::Dbo::dbo_default_traits.

template<class C>
static const char* Wt::Dbo::dbo_traits< C >::surrogateIdField (  ) [static]

Configures the surrogate primary key field.

Returns the field name which is the surrogate primary key, corresponding to the object's id.

You can disable this auto-incrementing surrogate id by returning 0 instead. In that case you will need to define a natural id for your class using Wt::Dbo::id().

Reimplemented from Wt::Dbo::dbo_default_traits.

template<class C>
static const char* Wt::Dbo::dbo_traits< C >::versionField (  ) [static]

Configures the optimistic concurrency version field.

Optimistic concurrency locking is used to detect concurrent updates by an object from multiple sessions. On each update, the version of a record is at the same time checked (to see if it matches the version of the record that was read), and incremented. A StaleObjectException is thrown if a record was modified by another session since it was read.

This method must return the database field name used for this version field.

You can disable optimistic locking using a version field all together for your class by returning 0 instead.

Reimplemented from Wt::Dbo::dbo_default_traits.

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