Drop down of theExtCategory id

Dear all,

In the ProjTable form ,I have a setup tab with a grid where it has column of ExtCateogryID and ExtCategoryName … like the table which has ProjId , ExtCategoryId,ExtCategoryName as fields. Here the ExtCategoryID ,Name,projid are all inserted initially

IThere is another tab which is called mapping.It has ExtCategoryID,ExCateogyrNAme as fields. In the ExtCAtegoryId field i want to have a drop down list of all ExtCategoryId’s on click of the ExtCategoryID field.These Id should b with respect to projects. It should not display all ID. It should display only the ID with respective to the selected projectid.

I started with creating EDT for the ExtCategoryId. And made a relation of ExtCategoryId . After creating a relation ,in the projTableForm ,the drop down appears with all the ExtCategoryId.but it is not appearing with selection projid extID. Please let me know how to create a relation for to appear by the selected project related category id.

Let me know if am not clear.