The widgets used for data input and output in a (CRUD) data management user interface (UI) normally correspond to properties defined in a model class of an app. We have to distinguish between (various types of) input fields corresponding to (various kinds of) attributes, and choice widgets (such as selection lists) corresponding to enumeration attributes or to reference properties. Representing reference properties in the UI with
select controls, instead of
input fields, prevents the user from entering invalid ID references, so it takes care of referential integrity.
In general, a single-valued reference property can be rendered as a single-selection list in the UI, no matter how many objects populate the reference property's range, from which one specific choice is to be made. If the cardinality of the reference property's range is sufficiently small (say, not greater than 7), then we can also use a radio button group instead of a selection list.
A multi-valued reference property can be rendered as a multiple-selection list in the UI. However, the corresponding multiple-
select control of HTML is not really usable as soon as there are many (say, more than 20) different options to choose from because the way it renders the choice is visually too scattered. In the special case of having only a few (say, no more than 7) options, we can also use a checkbox group instead of a multiple-selection list. But for the general case of having in the UI a list containing all associated objects chosen from the reference property's range class, we need to develop a special UI widget that allows to add (and remove) objects to (and from) a list of chosen objects.
Such a multiple-choice widget consists of
an HTML list element containing the chosen (associated) objects, where each list item contains a push button for removing the object from the choice;
select control that, in combination with a push button, allows to add a new associated object from the range of the multi-valued reference property.