Generate a discount code when a certain product is purchased, with Mechanic.

shopify/orders/paid
mechanic/actions/perform

{% assign required_product_id = options.required_product_id__number_required %}
{% assign discount_collection_id = options.discount_collection_id__number %}
{% assign discount_code_prefix = options.discount_code_prefix %}
{% assign discount_fixed_amount = options.discount_fixed_amount__number %}
{% assign discount_percentage = options.discount_percentage__number %}
{% assign discount_applies_to_each_line_item_individually = options.discount_applies_to_each_line_item_individually__boolean %}
{% assign discount_lifetime_in_days = options.discount_lifetime_in_days__number %}
{% assign discount_can_be_used_by_anyone = options.discount_can_be_used_by_anyone__boolean %}
{% assign email_subject = options.email_subject__required %}
{% assign email_body = options.email_body__multiline_required %}

{% if discount_percentage == blank and discount_fixed_amount == blank %}
  {% error "Please fill either the discount percentage or discount fixed amount." %}

{% elsif discount_percentage != blank and discount_fixed_amount != blank %}
  {% error "Please choose between the discount percentage and discount fixed amount - only one is permitted." %}
{% endif %}

{% if discount_collection_id != blank %}
  {% assign discount_collection_id = discount_collection_id | prepend: "gid://shopify/Collection/" %}
{% endif %}

{% if event.topic == "shopify/orders/paid" %}
  {% if event.preview %}
    {% capture order_json %}
      {
        "email": "customer@example.com",
        "customer": {
          "id": 1234567890
        },
        "line_items": [
          {
            "id": 1234567890,
            "product_id": {{ required_product_id }},
            "quantity": 1
          }
        ]
      }
    {% endcapture %}

    {% assign order = order_json | parse_json %}
  {% endif %}

  {% if order.customer == blank %}
    {% log "This order has no related customer. Skipping" %}
    {% break %}
  {% endif %}

  {% if order.email == blank %}
    {% log "This order has no email address. Skipping" %}
    {% break %}
  {% endif %}

  {% assign customer_id = order.customer.id | prepend: "gid://shopify/Customer/" %}

  {% comment %}
    -- create a discount code for each quantity of matching line items
  {% endcomment %}

  {% for line_item in order.line_items %}
    {% if line_item.product_id != required_product_id %}
      {% continue %}
    {% endif %}

    {% for n in (1..line_item.quantity) %}
      {% assign discount_code = line_item.id | append: n | split: "" | reverse | join: "" | slice: 0, 4 | append: order.order_number | base64 | replace: "=", "" | upcase | prepend: discount_code_prefix %}

      {% action "shopify" %}
        mutation {
          discountCodeBasicCreate(
            basicCodeDiscount: {
              code: {{ discount_code | json }}
              customerSelection: {
                {% if discount_can_be_used_by_anyone %}
                  all: true
                {% else %}
                  customers: {
                    add: {{ array | push: customer_id | json }}
                  }
                {% endif %}
              }
              title: {{ discount_code | json }}
              startsAt: {{ "now" | date: "%FT%T%:z" | json }}
              {% if discount_lifetime_in_days != blank %}
                {% assign validity_days_s = 60 | times: 60 | times: 24 | times: discount_lifetime_in_days %}
                endsAt: {{ "now" | date: "%s" | plus: validity_days_s | date: "%FT%T%:z" | json }}
              {% endif %}
              customerGets: {
                items: {
                  {% if discount_collection_id != blank %}
                    collections: {
                      add: {{ array | push: discount_collection_id | json }}
                    }
                  {% else %}
                    all: true
                  {% endif %}
                }
                value: {
                  {% if discount_percentage != blank %}
                    percentage: {{ discount_percentage | abs | divided_by: 100.0 | json }}
                  {% else %}
                    discountAmount: {
                      amount: {{ discount_fixed_amount | abs | times: 1.0 | json }}
                      {% if discount_collection_id != blank %}
                        appliesOnEachItem: {{ discount_applies_to_each_line_item_individually }}
                      {% else %}
                        appliesOnEachItem: false
                      {% endif %}
                    }
                  {% endif %}
                }
              }
            }
          ) {
            codeDiscountNode {
              id
              codeDiscount {
                ... on DiscountCodeBasic {
                  codes(first: 1) {
                    nodes {
                      id
                      code
                    }
                  }
                  endsAt
                  summary
                }
              }
            }
            userErrors {
              code
              extraInfo
              field
              message
            }
          }
        }
      {% endaction %}
    {% endfor %}
  {% endfor %}

{% elsif event.topic == "mechanic/actions/perform" %}
  {% comment %}
    -- only process a successful discountCodeBasicCreate shopify action; the Error reporter task should be used to respond to failures
  {% endcomment %}

  {% unless action.type == "shopify" and action.run.ok == true %}
    {% break %}
  {% endunless %}

  {% comment %}
    -- send a customer email for the created discount code
  {% endcomment %}

  {% assign discount_code = action.run.result.data.discountCodeBasicCreate.codeDiscountNode.codeDiscount.codes.nodes.first.code %}

  {% assign email_subject = email_subject | replace: 'DISCOUNT_CODE', discount_code %}
  {% assign email_body = email_body | replace: 'DISCOUNT_CODE', discount_code %}

  {% action "email" %}
    {
      "to": {{ event.parent.data.email | json }},
      "subject": {{ email_subject | strip | json }},
      "body": {{ email_body | strip | newline_to_br | json }},
      "reply_to": {{ shop.customer_email | json }},
      "from_display_name": {{ shop.name | json }}
    }
  {% endaction %}
{% endif %}