Controller is not able to pass categoryId to my DB

Hi,

Check what value you are getting inside the data variable.

var data - JSON.stringify({
productModel: productModel
});

If you are getting a null value after productModel.tblCategory after the stringify, then try to initialize the object before assigning the category value into the tblCategory object. like the following way.

productModel.tblCategory = new Object(),
productModel.tblCategory = category,