Question

I have a table which PolygonMapping whose register contains a polygon id and other ids of other related tables.

The Polygon table only contains its ID.

There is also the line table, whose register contains the polygon_ID it is part of and a couple of points_Id (start point and end point).

The point table contains 2 coordinates only (X and Y).

I am confused about how to map this database structure using NHibernate for C#. I would like to be able to easily access the lines of a polygon (so I think having a list of lines in the polygon class would be good), and I would like to able to save only the PolygonMapping class when I want to update one point, line or polygon. I would like to have it done automatically.

Please help me!

Thank you.

Answer 1

I would map this problem by creating three domain model objects: a Point object that describes a point, a Line object that contains a two Point objects, one named “StartPoint” and one named “EndPoint”, and a Polygon object that contains an IEnumerable of Lines. The domain objects would look like this:

public class Point
{
    public int Id { get; set; }
    public int XVal {get; set;}
    public int YVal {get; set;}
}

public class Line
{
    public int Id {get; set;}
    public Point StartPoint {get; set;}
    public Point EndPoint {get; set;}
}
public class Polygon
{
    public Polygon()
    {
        Lines = new HashSet<Line>();
    }
    public int Id {get; set;}
    public string Description { get; set; }
    public ICollection<Line> Lines { get; set; }
}

You could persist this class using a database schema that has a table for each domain model object.

The SQL DDL to create this database structure is as follows:

create table Point
(
    PointId int primary key identity(1, 1),
    XVal int,
    YVal int
)

create table Polygon
(
    PolygonId int primary key identity(1, 1),
[Description] nvarchar(255)
)

create table Line
(
    LineId int primary key identity(1, 1),
    PolygonId int foreign key references Polygon(PolygonId),
    StartPointId int foreign key references Point(PointId),
    EndPointId int foreign key references Point(PointId)
)

Your final task is to write your nHibernate mapping file to map the domain model to the underlying database tables. This can be done as shown below. Note that I set the “cascade” attributes to “all” to meet your requirement that saving the parent Polygon object cascades the changes to the child objects.

  <class name="Polygon" table="Polygon" lazy="false" >
    <id name="Id" column="PolygonId">
      <generator class="identity" />
    </id>
    <property name="Description" column="Description" />
    <set name="Lines" table="Line" lazy="false" cascade="all">
      <key column="PolygonId" />
      <one-to-many class="Line"  />
    </set>
  </class>

  <class name="Line" table="Line" lazy="false">
    <id name="Id" column="LineId">
      <generator class="identity" />
    </id>
    <many-to-one name="StartPoint" column="StartPointId" class="Point" cascade="all"/>
    <many-to-one name="EndPoint" column="EndPointId" class="Point" cascade="all"/>
  </class>
</hibernate-mapping>

With this mapping you can manipulate your parent Polygon object and the entire object graph will be persisted to the database when it is saved. For example, to add a new Line to a Polygon object, you can use the following code snippet:

        using (var session = factory.OpenSession())
        using(var tran = session.BeginTransaction())
        {
           var newPoint = session.Get<Point>(5);
           var newPoint2 = session.Get<Point>(2);
           var newLine = new Line { StartPoint = newPoint, EndPoint = newPoint2 };
            var foo2 = session.Get<Polygon>(1);
            foo2.Lines.Add(newLine);
            session.SaveOrUpdate(foo2);
            tran.Commit();
         }

Edit:
The above mapping assumes that you always want to access Line objects only thru the parent Polygon object. If you want to access Lines directly, you may want to add a many-to-one reference from the Line object to the Polygon parent. To do this, you will need to add the following property to the Line class:

 public Polygon Polygon {get; set;}

as well adding the corresponding mapping in the Line mapping file:

<many-to-one class="Polygon" name="Polygon" lazy="false"  />

With these changes, you should now be able to directly load a Line object that contains it’s Polygon parent:

var line = session.Get<Line>(5);
var parent = line.Polygon;

Edit 2
Note that if you make the Polygon-Line association bidirectional, you will need to add code to your domain model to ensure graph consistency. See for example this SO post.