Wt::Dbo::dbo_traits< C > Class Template Reference

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

#include <Wt/Dbo/Dbo>

Inheritance diagram for Wt::Dbo::dbo_traits< C >:

Public Types
typedef YourIdType	IdType
	Type of the primary key. More...
Public Types inherited from Wt::Dbo::dbo_default_traits
typedef long long	IdType
	Type of the primary key. More...

Static Public Member Functions
static IdType	invalidId ()
	Returns the sentinel value for a null id. More...
static const char *	surrogateIdField ()
	Configures the surrogate primary key field. More...
static const char *	versionField ()
	Configures the optimistic concurrency version field. More...
Static Public Member Functions inherited from Wt::Dbo::dbo_default_traits
static IdType	invalidId ()
	Returns the sentinel value for a null id. More...
static const char *	surrogateIdField ()
	Returns the database field name for the surrogate primary key. More...
static const char *	versionField ()
	Configures the optimistic concurrency version field. More...

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> : dbo_default_traits
    {
       static const char *surrogateIdField() { return "foo_id"; }
    };

    // Necessary if you want to use ptr<const Foo>
    template<> struct dbo_traits<const Foo> : dbo_traits<Foo> {};
  }
}

Note

The safe pattern to define traits is before the class definition, based on a forward declaration. This is necessary since the persist() function relies on this specialization:

class Foo;

namespace Wt {
  namespace Dbo {
    template<> struct dbo_traits<Foo> : ... { };
  }
}

class Foo {
  // definition here, including the persist() function
};

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");
    }
  }
}

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.

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 nullptr instead. In that case you will need to define a natural id for your class using Wt::Dbo::id().

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 nullptr instead.