Creating master-detail table and dialog, how to reuse same dialog for create and edit

Even if we fix your problem, this construct

<p:inputText value="#{guestList.hasSelected() ? '' : guestList.selectedGuest.name}">

is not ever going to work. It needs to reference a model property, not an empty string.

You'd best just reuse the edit form and let the create button precreate an empty entity. This would simplify a lot in the view side. It'd be more easy if the entity has an @Id property which is only present when it's persisted in the database.

Here's a kickoff example:

<h:form id="entitiesForm">
    <p:dataTable id="entitiesTable" value="#{bean.entities}" var="entity">
        <p:column>#{entity.foo}</p:column>
        <p:column>#{entity.bar}</p:column>
        <p:column>
            <p:commandButton id="edit" value="Edit" 
                process="@this" action="#{bean.edit(entity)}"
                update=":entityDialog" oncomplete="PF('entityDialog').show()" />
            <p:commandButton id="delete" value="Delete" 
                process="@this" action="#{bean.delete(entity)}"
                update=":entitiesForm:entitiesTable" />
        </p:column>
    </p:dataTable>
    <p:commandButton id="add" value="Add" 
        process="@this" action="#{bean.add}" 
        update=":entityDialog" oncomplete="PF('entityDialog').show()" />
</h:form>

<p:dialog id="entityDialog" widgetVar="entityDialog" 
    header="#{empty bean.entity.id ? 'New' : 'Edit'} entity">
    <h:form id="entityForm">
        <p:inputText id="foo" value="#{bean.entity.foo}" />
        <p:inputText id="bar" value="#{bean.entity.bar}" />
        <p:commandButton id="save" value="#{empty bean.entity.id ? 'Create' : 'Update'} entity" 
            process="@form" action="#{bean.save}"
            update=":entitiesForm:entitiesTable" oncomplete="PF('entityDialog').hide()" />
    </h:form>
</p:dialog>

With this @ViewScoped bean:

private List<Entity> entities; // +getter
private Entity entity; // +getter

@EJB
private EntityService entityService;

@PostConstruct
public void load() {
    entities = entityService.list();
    entity = null;
}

public void add() {
    entity = new Entity();
}

public void edit(Entity entity) {
    this.entity = entity;
}

public void save() {
    entityService.save(entity); // if (id==null) em.persist() else em.merge()
    load();
}

public void delete(Entity entity) {
    entityService.delete(entity); // em.remove(em.find(type, id))
    load();
}

See also:

• Creating master-detail pages for entities, how to link them and which bean scope to choose
• Keep p:dialog open when a validation error occurs after submit