Criteria from multiple fields for lookup

Criteria from multiple fields for lookup

I know similar situations have been addressed, but I'm having trouble implementing a solution.

I am working with 4 forms right now.

Crops_form
Section_form
Planting_form
Harvest_form

One record in Crops_form is related to many records in Planting_form
One record in Section_form is related to many records in Planting_form
In the Harvest_form, there is 
a single-select look-up field "Crops_field" which looks to the Crop_form
a multiselect look-up field "Section_field" which looks to the Section_form
a multiselect look-up field "Planting_field" which looks to the Planting_form

I am designing the data entry for the Harvest_form

I am trying to create a script in deluge "on user input" upon Section_form look up, that will populate the Planting_field multiselect lookup with records that match the input for input.Section_field and input.Crops_field.

Please let me know if clarification is needed. Thank you.

This is what I am trying, but it is not working:
  1. variable = Planting_Transplant [((Section == input.Section) && (Crops == input.Crops))];
  2. Planting_Transplant:ui.add(variable.ID.getall());