.net – ADO.NET: Adding DataRelation to a DataSet; which is parent and which is child

ado.netdatabasenet

Consider an SQL Server table containing:

ID   ParentID   Text
===  =========  =============
1    (null)     Product
2    (null)     Applications
3    1          Background
4    1          Details 
5    2          Mobile

i fill a SqlDataSet with the table, and now i want to add the Parent-Child relation to the DataSet:

public DataRelation(
    string relationName,
    DataColumn parentColumn,
    DataColumn childColumn,
    bool createConstraints
)

Now this guy uses:

DataRelation relation = newDataRelation("ParentChild",
    ds.Tables[0].Columns["ID"], //parentColumn
    ds.Tables[0].Columns["ParentID"] //childColumn, 
    true //createConstraints
);

But when i do that i get the exception:

This constraint cannot be enabled 
as not all values have corresponding parent values.

People have suggested passing false for createConstraints; but then why does it work for him?

And what is a child and what is a parent anyway? i would have thought the child column is the column that needs pointing to a parent, and parent column is the thing that does the pointing, which would reverse the relation:

DataRelation relation = newDataRelation("ParentChild",
    ds.Tables[0].Columns["ParentID"], //parentColumn
    ds.Tables[0].Columns["ID"], //childColumn
    true //createConstraints
);

So which is it? Why does his work? What's with the exception? Why can he create the constraint when he has nulls?

God it's hot in here.

Best Answer

Wow no one had the right answer ....

The problem is that the example you were reading is under the label "Step 3 - Retrieve Data and Create Nested Relationships".

If you would like to add a relation between two columns of the SAME TABLE (nested), then you must set the 'Nested" variable to true(before adding it) as is shown on his website.

relation.Nested = true;
ds.Relations.Add(relation);
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