Главная страница | назад





Article #17258: Propagating constraints from middle-tier to client

 Question and Answer Database

FAQ2258D.txt - Propagating constraints from middle-tier to client

Category   :Multi-tier
Platform   :All-32Bit
Product    :

Question:
How do I propagate constraints from Middle-tier to Client?

Answer:
This example uses the demos found in 
\Delphi 3\Demos\DB\CLIENTDS\Empedit.dpr and \server.dpr.  
Empedit.dpr represents the client tier, server.dpr runs on 
the middle tier, and the database, DBDEMOS, is located also on 
the middle tier, but in a real world scenario would be an SQL 
database running on yet a third machine.  This example uses the 
Employee.db table.  In order to use the following, you will need 
to set up DCOM and have the two examples running before adding 
the complication of constraints to the mix.  

The general steps are:

Define constraints in the data dictionary
Associate the data dictionary attributes to a TField
Access the TField from a ClientDataSet.  

First, define an attribute set in the data dictionary that 
includes a constraint.  For instance, an attribute set called 
EmpNoA could have a CustomConstraint of "EmpNo > 100" and a 
ConstraintErrorMessage of "EmpNo is less than 100". Define 
static TFields in Server.dpr.  Bring up the EmpServer datamodule 
and double click EmpQuery to bring up the fields editor. In the 
fields editor, right-click the EmpNo TField, select Associate 
Attributes, and select EmpNoA from the Associate Attributes 
dialog.  Now when selecting the EmpNo TField in the fields 
editor you should see CustomConstraint and 
ConstraintErrorMessage matching the data dictionary entry. Open 
EmpEdit.dpr and add a TDBEdit to EmployeeForm.  Set the 
DataField property to EmpNo and the DataSource property to 
EmpData (the TClientDataSet). Compile and run Server.dpr on the 
middle tier machine, then on the client machine run the EmpEdit 
example application.  Attempt to post a record where EmpNo is 
greater than 100; your custom error message will appear.

4/2/99 1:29:56 PM
 

Last Modified: 01-SEP-99