Problem with Lists in JSF & JPA

I do not know if the way I'm doing is correct, but I have a problem:

  

java.lang.IllegalArgumentException: id to load is required for   loading.

When I try to remove a template from the list, and in addition, the list of selected templates, listaCompra when displaying in xhtml , does not display template information.

@ManagedBean(name = "RegistrarCompra")
@SessionScoped
public class RegistrarCompraRegra {

    private Compra compra;
    private Template template;
    private Compra_DAO cDao;
    private Collection<Template> homeTemplate;
    private int numeroTemplates;

    public RegistrarCompraRegra() {
        compra = new Compra();

    }

    public String novo() {
        compra = new Compra();
        return null;
    }

    public String gravar() {
        FacesContext fc = FacesContext.getCurrentInstance();
        FacesMessage fm;

        cDao = new Compra_DAO();
        if (this.compra.getCodCompra() == null) {
            try {
                cDao.gravar(compra);
                fm = new FacesMessage("Sucesso!");
                fc.addMessage(null, fm);
            } catch (Exception ex) {
                fm = new FacesMessage(ex.getMessage());
                fc.addMessage(null, fm);

            }
        } else {

            cDao.alterar(compra);
            fm = new FacesMessage("Alteração realizada com sucesso!");
            fc.addMessage(null, fm);

        }
        return "Carrinho";
    }

    public String adicionaCarrinho() {
        this.numeroTemplates +=1;
        this.template = new Template();
        this.compra.getListaCompra().add(template);
        return null;
    }

    public String removeCarrinho() {
        this.numeroTemplates -=1;
        Compra_DAO cDAO = new Compra_DAO();
        compra.getListaCompra().remove(template);
        this.compra = cDAO.atualizar(this.compra);
        return null;
    }
    public Collection<Template> getHomeTemplate() {
        Template_DAO tDAO = new Template_DAO();
        this.homeTemplate = tDAO.todos();
        return this.homeTemplate;
    }

//Getters e Setters & Hash


@Entity
@Table(name = "Compra")
@SequenceGenerator(initialValue = 1, name = "Gerador_codCompra", sequenceName = "Gerador_codCompra", allocationSize = 1)
public class Compra implements Serializable {

    private static final long serialVersionUID = -4846590792485118024L;
    @Id
    @GeneratedValue(generator = "Gerador_codCompra", strategy = GenerationType.SEQUENCE)
    private Long codCompra;

    @OneToMany(cascade = CascadeType.ALL)
    @JoinColumn(name = "codTemplate")
    private Collection<Template> listaCompra;

    @ManyToOne
    @JoinColumn(name = "codCliente")
    private Cliente cliente;

    public Compra() {
        this.listaCompra = new ArrayList<>();
    }

// XHTML

<h:dataTable value="#{RegistrarCompra.compra.listaCompra}" var="Carrinhotemplate" rules="rows" class="tabela" border="1">

                        <h:inputHidden value="#{Carrinhotemplate.codTemplate}" />


                        <h:column  id="Nome" class="cabecalho">
                            <f:facet name="header">
                                <h:outputText value="Nome"></h:outputText>
                            </f:facet>
                            <h:outputText value="#{Carrinhotemplate.nome}"></h:outputText>              
                        </h:column>



                        <h:column id="Categoria" class="cabecalho">
                            <f:facet name="header">
                                <h:outputText value="Categoria"></h:outputText>
                            </f:facet>
                            <h:outputText value="#{Carrinhotemplate.categoria}" />
                        </h:column>


                        <h:column id="Valor" class="cabecalho">
                            <f:facet name="header">
                                <h:outputText value="Valor"></h:outputText>
                            </f:facet>
                            <h:outputText value="#{Carrinhotemplate.valor}" />
                        </h:column>


                        <h:column id="column4" class="ultimo">    
                            <h:commandButton action="#{RegistrarCompra.removeCarrinho()}" value="Remover do Carrinho">
                                <f:setPropertyActionListener target="#{RegistrarCompra.template}" value="#{template}"/>
                            </h:commandButton>
                        </h:column>