How Primary Keys are created

By default wwBusinessObject works with integer PKs and uses a lookup table to generate new ids as objects are created. The default process works like this:

When you call the New() method:

  • New() creates a new blank record and issues a SCATTER NAME on it
  • New() calls CreateNewId() to generate a new PK and assigns it to the PK field
  • CreateNewId() internally calls a VFP Function or SQL Server Stored Procedure to generate a new Integer PK
  • You can override CreateNewId() to return a custom value with your own logic

How the default New Id Creation behavior works

The key here is the CreateNewId() method which generates the new Integer Pk. This default behavior uses a lookup table which contains a key for the table name and an integer value that is incremented. When a new Id is requested the single record is briefly locked and the value incremented by one and returned.

FoxPro New Id Creation

In FoxPro this behavior is handled by FoxPro code inside of CreateNewId() using the table defined in the cIDTable property.

Overriding New Id Generation

You can easily override or completely remove the ID generation logic. The CreateNewId() method is responsible for generating new ids and it returns a result value that is used for the new ID when CreateNewId() is called.

To override the ID generation just override the method and return whatever value and type you like. For example to generate new string Ids you might use:

FUNCTION CreateNewId()

*** Built-in
* RETURN SYS(2015)

*** Generate a 12 character Unique ID from wwutils.prg
RETURN GetUniqueId(10)
ENDFUNC

The resulting value is assigned to the PK field.

If you want to skip ID generation because your database automatically creates new auto-ids you can RETURN -1:

FUNCTION CreateNewId()
RETURN -1

This skips the assignment part of the new ID generation. Note that you'll be responsible for filling the .oData.Pk (or whatever field you use for a PK) if you use that most likely by reloading the record after it's been saved with the generated id.

One advantage of the pre-generation of keys is that you can assign a newly generated key to a relationship that has not been saved to disk yet.

Sql Server New Id Creation

For SQL Server a stored procedure sp_ww_NewId is used and it relies on a table called wws_id to hold the tables:

CREATE PROCEDURE [dbo].[sp_ww_NewID]
  @cName char(30),
  @nRetval int OUTPUT
AS
UPDATE wws_Id 
  SET id = id + 1,
      @nRetval = id + 1
WHERE TableName = @cName

if @@ROWCOUNT < 1
BEGIN
	insert into wws_id (Tablename,Id,pkfield,pktype,pkwidth) values (@cName,1,'','',0)
		set @nRetval = 1
END

The table and stored procedure can be automatically generated from the Console using the Create Sql Server Tables.

The wws_id table is very simple and looks like this:

CREATE TABLE [dbo].[wws_id](
	[tablename] [varchar](50) NOT NULL,
	[id] [int] NOT NULL,
	[pkfield] [varchar(50)] NOT NULL CONSTRAINT [df_wwsid_pkfield] DEFAULT '',
	[pkwidth] [int] NOT NULL CONSTRAINT [df_wwsid_pkwidth] DEFAULT 0,
	[pktype] [varchar(5)] NOT NULL CONSTRAINT [df_wwsid_pktype] DEFAULT ''
) ON [PRIMARY]

Each table that you want to create a PK for should have an entry in tablename field and id should start at 0.


© West Wind Technologies, 1996-2019 • Updated: 02/17/19
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