Tài liệu Creating and Using a DataRelation Object doc

Once you've created a DataRelation, you can use the GetChildRows method of a DataRow object in the parent DataTable to obtain the corresponding DataRow objects from the child DataTable..

Creating and Using a DataRelation Object

In this section, you'll learn how to create a DataRelation object to define a relationship between two DataTable objects that hold some rows from the Customers and Orders tables As you know, the CustomerID column of the child Orders table is a foreign key that links to the CustomerID column of the parent Customers table

Once you've created a DataRelation, you can use the GetChildRows() method of a

DataRow object in the parent DataTable to obtain the corresponding DataRow objects from the child DataTable By "corresponding," I mean the rows that have matching values in the foreign key DataColumn objects You can also use the GetParentRow() method of a DataRow in the child DataTable to obtain the corresponding DataRow in the parent DataTable

Before creating and adding a DataRelation to a DataSet, you first need a DataSet The following example creates and populates a DataSet with two DataTable objects named customersDT and ordersDT; notice that the top two rows from the Customers table along with the corresponding rows from the Orders table are retrieved:

SqlCommand mySqlCommand = mySqlConnection.CreateCommand();

mySqlCommand.CommandText =

"SELECT TOP 2 CustomerID, CompanyName " +

"FROM Customers " +

"ORDER BY CustomerID;" +

"SELECT OrderID, CustomerID " +

"FROM Orders " +

"WHERE CustomerID IN (" +

" SELECT TOP 2 CustomerID " +

" FROM Customers " +

" ORDER BY CustomerID" +

")";

SqlDataAdapter mySqlDataAdapter = new SqlDataAdapter();

mySqlDataAdapter.SelectCommand = mySqlCommand;

DataSet myDataSet = new DataSet();

mySqlConnection.Open();

mySqlDataAdapter.Fill(myDataSet);

mySqlConnection.Close();

myDataSet.Tables["Table"].TableName = "Customers";

myDataSet.Tables["Table1"].TableName = "Orders";

DataTable customersDT = myDataSet.Tables["Customers"];

DataTable ordersDT = myDataSet.Tables["Orders"];

You'll see how to create a DataRelation that defines a relationship between the

customersDT and ordersDT DataTable objects next

Note You'll find all the code examples shown in this section in the

CreateDataRelation.cs program

Creating the DataRelation

The DataRelation constructor is overloaded as follows:

DataRelation(string dataRelationName,

DataColumn parentDataColumn, DataColumn childDataColumn)

DataRelation(string dataRelationName,

DataColumn[] parentDataColumns, DataColumn[] childDataColumns)

DataRelation(string dataRelationName,

DataColumn parentDataColumn, DataColumn childDataColumn, bool

createConstraints)

DataRelation(string dataRelationName,

DataColumn[] parentDataColumns, DataColumn[] childDataColumns,

bool createConstraints)

DataRelation(string dataRelationName,

string parentDataTableName, string childDataTableName,

string[] parentDataColumnNames, string[] childDataColumnNames,

bool nested)

where

• dataRelationName is the name you want to assign to the RelationName property

of your DataRelation

• parentDataColumn and parentDataColumnsare the DataColumn objects in the

parent DataTable

• childDataColumn and childDataColumns are the DataColumn objects in the

child DataTable

• createConstraints indicates whether you want a UniqueConstraint added to the

parent DataTable and a ForeignKeyConstraint added to the child DataTable

automatically (the default is true)

• parentDataTableName and childDataTableName are the names of the parent and

child DataTable objects

• parentDataColumnNames and childDataColumnNames contain the names of the

DataColumn objects in the parent and child DataTable objects

• nested indicates whether the relationships are nested

The following example creates a DataRelation object named customersOrdersDataRel:

DataRelation customersOrdersDataRel =

new DataRelation(

"CustomersOrders",

customersDT.Columns["CustomerID"],

ordersDT.Columns["CustomerID"]

);

The name assigned to the RelationName property of customersOrdersDataRel is

CustomersOrders, the parent DataColumn is customersDT.Columns["CustomerID"], and the child DataColumn is ordersDT.Columns["CustomerID"]

Next, customersOrdersDataRel must be added to myDataSet You access the

DataRelation objects in a DataSet object through its Relationships property The

Relationships property returns an object of the DataRelationCollection class, which is a collection of DataRelation objects To add a DataRelation object to the

DataRelationCollection object of a DataSet, you call the Add() method through the Relationships property of your DataSet

The following example uses the Add() method to add customersOrdersDataRel to

myDataSet:

myDataSet.Relations.Add(

customersOrdersDataRel

);

The Add() method is overloaded, and you can also use the following version of the Add() method to create and add a DataRelation object to myDataSet:

myDataSet.Relations.Add(

"CustomersOrders",

customersDT.Columns["CustomerID"],

ordersDT.Columns["CustomerID"]

);

This example does the same thing as the two earlier examples The first parameter to the Add() method is a string containing the name you want to assign to the RelationName property of the DataRelation The second and third parameters of the relationship are the DataColumn objects from the parent and child DataTable objects

Examining the Constraints Created by the DataRelation

By default, when you create a DataRelation, a UniqueConstraint and

ForeignKeyConstraint are automatically added to your parent and child DataTable

objects You can get the UniqueConstraint from a DataRelation using its

ParentKeyConstraint property For example:

UniqueConstraint myUC =

customersOrdersDataRel.ParentKeyConstraint;

You can view the properties of the myUC UniqueConstraint object using the following code:

Console.WriteLine("Columns:");

foreach (DataColumn myDataColumn in myUC.Columns)

{

Console.WriteLine("" + myDataColumn);

}

Console.WriteLine("myUC.ConstraintName = " + myUC.ConstraintName);

Console.WriteLine("myUC.IsPrimaryKey = " + myUC.IsPrimaryKey);

Console.WriteLine("myUC.Table = " + myUC.Table);

The output from this code is as follows:

Columns:

CustomerID

myUC.ConstraintName = Constraint1

myUC.IsPrimaryKey = False

myUC.Table = Customers

You can get the ForeignKeyConstraint from a DataRelation using its ChildKeyConstraint property For example:

ForeignKeyConstraint myFKC =

customersOrdersDataRel.ChildKeyConstraint;

You can view the properties of myFKC using the following code:

Console.WriteLine("myFKC.AcceptRejectRule = " + myFKC.AcceptRejectRule);

Console.WriteLine("Columns:");

foreach (DataColumn myDataColumn in myFKC.Columns)

{

Console.WriteLine(""+ myDataColumn);

}

Console.WriteLine("myFKC.ConstraintName = " + myFKC.ConstraintName);

Console.WriteLine("myFKC.DeleteRule = " + myFKC.DeleteRule);

Console.WriteLine("RelatedColumns:");

foreach (DataColumn relatedDataColumn in myFKC.RelatedColumns)

{

Console.WriteLine("" + relatedDataColumn);

}

Console.WriteLine("myFKC.RelatedTable = " + myFKC.RelatedTable);

Console.WriteLine("myFKC.Table = " + myFKC.Table);

Console.WriteLine("myFKC.UpdateRule = " + myFKC.UpdateRule);

The output from this code is as follows:

myFKC.AcceptRejectRule = None

Columns:

CustomerID

myFKC.ConstraintName = CustomersOrders

myFKC.DeleteRule = Cascade

RelatedColumns:

CustomerID

myFKC.RelatedTable = Customers

myFKC.Table = Orders

myFKC.UpdateRule = Cascade

The DeleteRule and UpdateRule properties are set to Cascade by default Because

DeleteRule is set to Cascade, when you delete a DataRow in the parent DataTable, then any corresponding DataRow objects in the child DataTable are also deleted Because UpdateRule is set to Cascade, when you change the DataColumn in the parent DataTable

on which the ForeignKeyConstraint was created, then the same change is also made in any corresponding DataRow objects in the child DataTable You'll learn more about this later in the section "Issues When Updating the Primary Key of a Parent Row."

Navigating DataRow Objects in the Parent and Child DataTable Objects

To navigate the DataRow objects in related DataTable objects, you use the

GetChildRows() and GetParentRows() methods of a DataRow

Using the GetChildRows() Method

You use the GetChildRows() method to get the related child DataRow objects from the parent DataRow For example, the following code displays the parent DataRow objects from the customersDT DataTable and their related child DataRow objects from the ordersDT DataTable:

foreach (DataRow customerDR in customersDT.Rows)

{

Console.WriteLine("\nCustomerID = " + customerDR["CustomerID"]);

Console.WriteLine("CompanyName = " + customerDR["CompanyName"]);

DataRow[] ordersDRs = customerDR.GetChildRows("CustomersOrders");

Console.WriteLine("This customer placed the following orders:");

foreach (DataRow orderDR in ordersDRs)

{

Console.WriteLine("OrderID = " + orderDR["OrderID"]);

}

}

The output from this code is as follows:

CustomerID = ALFKI

CompanyName = Alfreds Futterkiste

This customer placed the following orders:

OrderID = 10643

OrderID = 10692

OrderID = 10702

OrderID = 10835

OrderID = 10952

OrderID = 11011

CustomerID = ANATR

CompanyName = Ana Trujillo Emparedados y helados

This customer placed the following orders:

OrderID = 10308

OrderID = 10625

OrderID = 10759

OrderID = 10926

Using the GetParentRow() Method

You use the GetParentRow() method to get the parent DataRow from the child DataRow For example, the following code displays the first child DataRow from ordersDT and its related parent DataRow from customersDT:

DataRow parentCustomerDR = ordersDT.Rows[0].GetParentRow("CustomersOrders"); Console.WriteLine("\nOrder with OrderID of " + ordersDT.Rows[0]["OrderID"] + " was placed by the following customer:");

Console.WriteLine("CustomerID = " + parentCustomerDR["CustomerID"]);

The output from this code is as follows:

Order with OrderID of 10643 was placed by the following customer: CustomerID = ALFKI