Lock 'Childreen' Table

Hi!

I'm new in SQL and I′ve been the problem bellow:

I lock a record in one foreign table and automaticaly SQL lock the record that matches on primary table.

But this occours just in some primaries tables and not in all. I need that just the table that are of SELECT are lock. How can I do this?

Example:

** At open of the invoice:

SET ISOLATION LEVEL READ UNCOMMITTED

BEGIN TRANSACTION

SELECT * FROM INVOICE WITH (ROWLOCK UPLOCK) WHERE ( ID = 15 )

******* at the end of invoice:

INSERT INTO INVOICE ........

COMMIT TRANSACTION

END

******* The relationship are:

INVOICE <>> ITENS_INVOICE

ITENS_INVOICE <>> PRODUCTS

CUSTOMER <>> INVOICE

VENDORS <>> INVOICE

Just the INVOICE and PRODUCTS record′s involved in Transaction are lock. ( The CUSTOMER and VENDORS are not locked for example )

But I need that just INVOICE record be locked.

Thank′s for all and sorry my English.

Igor Sane

S?o Paulo - Brazil

PS: This doesn′t occours in SQL Express, just in SQL Server 2005....