How get product IDs of root category and subcategory under root category programmatically?

Question

I want to get all product IDs of a root category and subcategory programmatically, when I click the root category. How can I get product IDs of both categories together under root category?

Answer 1

In admin panel set yes the value for Is Anchor.

Steps:

Go to Catalog > Manage Categories and set Is Anchor to Yes.

Re-index your catalog product index