Puting the value from a combo box into a subform

[John thomas]

Hi I have a form with a subform .Ihave cascading combo boxes in the form but want to enter the result from the combo box into a field on the subform. I.have tried setting the row source to that of the subform by using the expression builder .but when i try it out i get a message saying cant edit field
What am i doing wrong please

 

[JHB]

Because you lock the field when you put the ControlSource for the field to combobox row source.
You must transfer the value of your combobox to the field after the combo box has been updated. Place the below code in your combobox "AfterUpdate" event.

  Me.[YourSubformName].Form![YourFieldNameOnTheSubForm] = Me.[YourComboBoxName]