Magento – Magento 2 REST API: Get list and details of all active coupons

The coupon codes and shopping cart rules in general (even if they don't have a coupon code) have meaning relative to a cart (quote) object. They ca depend on different qtys in the cart, they can depend on shipping and billing addresses, or even on shipping and payment methods.
So you have to involve a quote in the process of validating a rule.
Magento has something for getting the valid rules depending on a quote and a coupon code.
See Mage_SalesRule_Model_Validator::init.
Inside that method there is this code that (I think) it filters the valid rules depending on the coupon code, website id and customer group.

$this->_rules[$key] = Mage::getResourceModel('salesrule/rule_collection')
            ->setValidationFilter($websiteId, $customerGroupId, $couponCode)
            ->load();

The code that actually retrieves these rules is located in Mage_SalesRule_Model_Resource_Rule_Collection::setValidationFilter.
This method receives 4 parameters.

• the website id - You already have that one
• the customer group - you already have that one
• the coupon code - this is what you need.
• a date that defaults to now - you have that one.

So my idea is to build your own method that is similar to the setValidationFilter but remove from that the conditions for the coupon codes.
You will end up with a set of rules that can be valid if you have the proper coupon code.
Then just loop through the rules you get and check which ones have coupon codes.
Those are the coupon codes you are looking for.
[EDIT]
Fogot one thing. The method I mentioned does not involve a quote. It just filters a bit the rules. You will still have to apply the coupons to a quote and see if they are valid, but at least you eliminate some of the rules.

The below code will help you achieve this, as it is implementing new custom API to get all products against the given category using the getAssignedProducts function where you are not all getting products but the complete product information of your choice.

Step 1 – Create webapi.xml under your custom module \Scommerce\Custom\etc\

<route url="/V1/custom/:categoryId/products" method="GET">
    <service class="Scommerce\Custom\Api\CategoryLinkManagementInterface" method="getAssignedProducts" />
    <resources>
        <resource ref="self"/>
    </resources>
</route>

Step 2 – Lets now create the main interface file for our web API

CategoryLinkManagementInterface.php under Scommerce\Custom\Api\ as specified in webapi.xml in Step 1

namespace Scommerce\Custom\Api;

/**
 * @api
 */
interface CategoryLinkManagementInterface
{
    /**
     * Get products assigned to a category
     *
     * @param int $categoryId
     * @return \Scommerce\Custom\Api\Data\CategoryProductLinkInterface[]
     */
    public function getAssignedProducts($categoryId);
}

Step 3 – Based on the return parameter in Step 2, let’s create our data interface CategoryProductLinkInterface.php under \Scommerce\Custom\Api\Data\

namespace Scommerce\Custom\Api\Data;

/**
 * @api
 */
interface CategoryProductLinkInterface
{
    /**
     * @return string|null
     */
    public function getSku();

    /**
     * @param string $sku
     * @return $this
     */
    public function setSku($sku);

    /**
     * @return string|null
     */
    public function getName();

    /**
     * @param string $name
     * @return $this
     */
    public function setName($name);

    /**
     * @return float|null
     */
    public function getPrice();

    /**
     * @param float $price
     * @return $this
     */
    public function setPrice($price);

    /**
     * @return int|null
     */
    public function getPosition();

    /**
     * @param int $position
     * @return $this
     */
    public function setPosition($position);

    /**
     * @return string|null
     */
    public function getCategoryDescription();

    /**
     * @param string $description
     * @return $this
     */
    public function setCategoryDescription($description);
}

Step 4 – Now our interface files are created, let’s create our model classes where we can put the actual business logic, to do so we would need to specify this in our di.xml file under \Scommerce\Custom\etc\

<config ...>
    <preference for="Scommerce\Custom\Api\CategoryLinkManagementInterface" type="Scommerce\Custom\Model\CategoryLinkManagement" />
    <preference for="Scommerce\Custom\Api\Data\CategoryProductLinkInterface" type="Scommerce\Custom\Model\CategoryProductLink" />
</config>

Step 5 – Let’s create our first model class CategoryLinkManagement.php under Scommerce\Custom\Model\ as specified in di.xml

namespace Scommerce\Custom\Model;

/**
 * Class CategoryLinkManagement
 */
class CategoryLinkManagement implements \Scommerce\Custom\Api\CategoryLinkManagementInterface
{
    /**
     * @var \Magento\Catalog\Api\CategoryRepositoryInterface
     */
    protected $categoryRepository;

    /**
     * @var \Scommerce\Custom\Api\Data\CategoryProductLinkInterfaceFactory
     */
    protected $productLinkFactory;

    /**
     * CategoryLinkManagement constructor.
     *
     * @param \Magento\Catalog\Api\CategoryRepositoryInterface $categoryRepository
     * @param \Scommerce\Custom\Api\Data\CategoryProductLinkInterfaceFactory $productLinkFactory
     */
    public function __construct(
        \Magento\Catalog\Api\CategoryRepositoryInterface $categoryRepository,
        \Scommerce\Custom\Api\Data\CategoryProductLinkInterfaceFactory $productLinkFactory
    ) {
        $this->categoryRepository = $categoryRepository;
        $this->productLinkFactory = $productLinkFactory;
    }

    /**
     * {@inheritdoc}
     */
    public function getAssignedProducts($categoryId)
    {
        $category = $this->categoryRepository->get($categoryId);
        if (!$category->getIsActive()) {
            return [[
                'error' => true,
                'error_desc' => 'Category is disabled'
            ]];
        }
        $categoryDesc = $category->getDescription();

        /** @var \Magento\Catalog\Model\ResourceModel\Product\Collection $products */
        $products = $category->getProductCollection()
            ->addFieldToSelect('position')
            ->addFieldToSelect('name')
            ->addFieldToSelect('price');

        /** @var \Scommerce\Custom\Api\Data\CategoryProductLinkInterface[] $links */
        $links = [];

        /** @var \Magento\Catalog\Model\Product $product */
        foreach ($products->getItems() as $product) {
            /** @var \Scommerce\Custom\Api\Data\CategoryProductLinkInterface $link */
            $link = $this->productLinkFactory->create();
            $link->setSku($product->getSku())
                ->setName($product->getName())
                ->setPrice($product->getFinalPrice())
                ->setPosition($product->getData('cat_index_position'))
                ->setCategoryDescription($categoryDesc);
            $links[] = $link;
        }

        return $links;
    }
}

Step 6 – Lets now create our second model class CategoryProductLink.php under Scommerce\Custom\Model\ as specified in di.xml

namespace Scommerce\Custom\Model;

/**
 * @codeCoverageIgnore
 */
class CategoryProductLink implements \Scommerce\Custom\Api\Data\CategoryProductLinkInterface
{
    /**#@+
     * Constant for confirmation status
     */
    const KEY_SKU                   = 'sku';
    const KEY_NAME                  = 'name';
    const KEY_PRICE                 = 'price';
    const KEY_CATEGORY_DESC         = 'category_description';
    const KEY_POSITION              = 'position';
    /**#@-*/

    /**
     * {@inheritdoc}
     */
    public function getSku()
    {
        return $this->_get(self::KEY_SKU);
    }

    /**
     * {@inheritdoc}
     */
    public function getName()
    {
        return $this->_get(self::KEY_NAME);
    }

    /**
     * {@inheritdoc}
     */
    public function getPosition()
    {
        return $this->_get(self::KEY_POSITION);
    }

    /**
     * {@inheritdoc}
     */
    public function getPrice()
    {
        return $this->_get(self::KEY_PRICE);
    }

    /**
     * {@inheritdoc}
     */
    public function getCategoryDescription()
    {
        return $this->_get(self::KEY_CATEGORY_DESC);
    }

    /**
     * @param string $sku
     * @return $this
     */
    public function setSku($sku)
    {
        return $this->setData(self::KEY_SKU, $sku);
    }

    /**
     * @param string $name
     * @return $this
     */
    public function setName($name)
    {
        return $this->setData(self::KEY_NAME, $name);
    }

    /**
     * @param int $position
     * @return $this
     */
    public function setPosition($position)
    {
        return $this->setData(self::KEY_POSITION, $position);
    }

    /**
     * @param float $price
     * @return $this
     */
    public function setPrice($price)
    {
        return $this->setData(self::KEY_PRICE, $price);
    }

    /**
     * @param string $description
     * @return $this
     */
    public function setCategoryDescription($description)
    {
        return $this->setData(self::KEY_CATEGORY_DESC, $description);
    }

}

The above will allow us to call our first Magento 2 web API using the customer token. To retrieve admin or custom token, please have a look at Magento official token retrieval documentation

source: https://www.scommerce-mage.com/blog/magento-2-how-to-create-custom-api.html