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

Question

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.

Solution 1

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);

Solution 2

the parent-child relationship is also called a one-to-many relationship, where the 'one' is the parent and the 'many' is the child

the child has a column that holds the key of the parent (aka the "foreign key" column)

in your example, it probably does not like nulls for the parent id; passing false to avoid enabling constraints will likely make the error go away

Comments

[Mass Dot Net]

This is exactly what I needed to know -- MSDN wouldn't give me a straight answer, as usual. tyvm!