gistfile1.php

In InvoicingClients:
       
    /**
     * @var ArrayCollection
     *
     * @ORM\ManyToMany(targetEntity="Customer", mappedBy="invoicingClients")
     * @ORM\JoinTable(
     *      name="invoicing_clients_customers",
     *      joinColumns={@ORM\JoinColumn(name="client_id", referencedColumnName="id")},
     *      inverseJoinColumns={@ORM\JoinColumn(name="customer_id", referencedColumnName="id")}
     *  )
     */
    private $customers;

in Customer:

    /**
     * @var ArrayCollection
     * @ORM\ManyToMany(targetEntity="InvoicingClient", mappedBy="customers")
     */
    private $invoicingClients;