Associating two related models

Question

So I'm creating a plugin wherein a customer can have grocery lists.

I have two created models - GroceryList and GroceryListItem. A grocery list contains one or many grocery list items. I can fetch data using GET /V1/grocerylist that fetches grocery lists for the logged in costumer.

This is the example response:

// GET /V1/grocerylist
[
    {
        "list_name": "Daddy's grocery list",
        "items": [
            {
                "name": "shampoo"
            },
            {
                "name": "soap"
            }
        ]
    },
    {
        "list_name": "Mommy's grocery list",
        "items": [
            {
                "name": "corned beef"
            },
            {
                "name": "baby diapers"
            }
        ]
    }
]

This data, however, are seeded data. So, I haven't created an API endpoint for creating a grocery list (POST /V1/grocerylist). My model classes already contain getter and setter methods (together with the corresponding API data interface, resource model classes, collection classes -- all barebones, just to make it work), but I have no idea how to associate GroceryListItem to GroceryList, and I'm not even sure I'm doing getItems() method right. And I think it's not as simple as this --

$this->setData('grocery_list_item', $items);

since this is not a simple field in a record (e.g. name of the grocery list).

My POST /V1/grocerylist right now accepts this request body:

{
    "data" :{
        "list_name": "New grocery list",
        "items": [
            {
                "name": "new item 1"
            },
            {
                "name": "new item 2"
            }
        ]
    }
 }

My expected results are:

• Return what's mentioned in data attribute (for testing purposes only)
• Save the grocery list, together with the respective items to database

Here are some of the related files:

Model/GroceryListRepository.php

<?php   
class GroceryListRepository implements GroceryListRepositoryInterface
{
    /**
     * @param int $customerId
     *
     * @return \Grocer\GroceryList\Api\Data\GroceryListInterface[]
     */
    public function getList($customerId)
    {
        return $this->listCollectionFactory->create()->getItemsByColumnValue('customer_id', $customerId);
    }

    /**
     * @param int $customerId
     * @param \Grocer\GroceryList\Api\Data\GroceryListInterface $data
     */
    public function createList($customerId, $data)
    {
        // TODO: Create grocery list
    }

Model/GroceryList.php

<?php
/*...*/
use Grocer\GroceryList\Model\ResourceModel\GroceryListItem\CollectionFactory as ItemCollectionFactory;
/*...*/

class GroceryList extends AbstractModel implements GroceryListInterface
{

    /*...*/

    public function __construct(ItemCollectionFactory $itemCollectionFactory)
    {
        $this->itemCollectionFactory = $itemCollectionFactory;
    }

    /**
     * @param GroceryListItemsInterface[] $items
     * @return $this
     */
    public function setItems(array $items = null)
    {
        // TODO: Set grocery list items to grocery list
    }

    /**
     * @return GroceryListItemsInterface[]
     */
    public function getItems()
    {
        return $this->itemCollectionFactory->create()
            ->getItemsByColumnValue('list_id', $this->getId());
    }

    /*...*/

}

Please enlighten me on this matter. Thank you.