# Stub data

**Stub data** is hard-coded into a task, providing an unchanging source of data for [**previews**](https://learn.mechanic.dev/core/tasks/previews). It is an important tool when generating [**dynamic preview actions**](https://learn.mechanic.dev/core/tasks/previews/..#dynamic-preview-actions). Stub data may be used for user-defined variables, but may also override [**environment variables**](https://learn.mechanic.dev/core/tasks/code/environment-variables) as needed.

{% hint style="info" %}
For controlling preview event data (i.e. the values in `event.data`, and values found in [event subject variables](https://learn.mechanic.dev/core/code/environment-variables#event-subject-variables)), use [**defined preview events**](https://learn.mechanic.dev/core/tasks/previews/events) to cleanly specify these values *outside* of the task code.
{% endhint %}

## Stubbing Liquid variables

Most tasks make decisions based on the [Liquid variables](https://learn.mechanic.dev/core/tasks/code/environment-variables) automatically provided, making it a common practice to stub them during preview mode. Any and all Liquid variables may be replaced by stub data, including `event` and any [event subject variables](https://learn.mechanic.dev/core/code/environment-variables#event-subject-variables).

In simple cases, replacement objects may be constructed using the [assign](https://learn.mechanic.dev/platform/liquid/tags/assign) tag.

{% hint style="info" %}
The stub data in the following examples include an ID for the order, so as to generate a realistic tagsAdd mutation during preview mode.

Realistic preview actions are important for users and developers, but there's a functional importance for tagsAdd mutations in particular: in preview mode, Mechanic looks at the `id` argument in order to determine what kind of resource will be tagged, in order to determine what permissions this particular mutation requires. If you generate tagsAdd mutations during preview, make sure to use realistic ID values!
{% endhint %}

```liquid
{% if event.preview %}
  {% assign order = hash %}
  {% assign order["source_name"] = "web" %}
  {% assign order["admin_graphql_api_id"] = "gid://shopify/Order/1234567890" %}
{% endif %}

{% if order.source_name == "web" %}
  {% action "shopify" %}
    mutation {
      tagsAdd(id: {{ order.admin_graphql_api_id | json }}, tags: "web") {
        userErrors { field, message }
      }
    }
  {% endaction %}
{% endif %}
```

It's also possible to construct this data using [parse\_json](https://learn.mechanic.dev/platform/liquid/filters#json-parse_json-parse_jsonl).

```liquid
{% if event.preview %}
  {% capture order_json %}
    {
      "source_name": "web",
      "admin_graphql_api_id": "gid://shopify/Order/1234567890"
    }
  {% endcapture %}

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

{% if order.source_name == "web" %}
  {% action "shopify" %}
    mutation {
      tagsAdd(id: {{ order.admin_graphql_api_id | json }}, tags: "web") {
        userErrors { field, message }
      }
    }
  {% endaction %}
{% endif %}
```

## Stubbing GraphQL data

Mechanic makes GraphQL data available to tasks via the [shopify](https://learn.mechanic.dev/platform/liquid/filters#shopify) filter. Mechanic observes the shopify filter in action during preview mode, using its inputs to inform Mechanic's knowledge of what permissions the task needs.

For this reason, it's important to allow the shopify filter to run normally, and construct stub data afterwards.

It can be useful to specify stub data using JSON, fed through the [parse\_json](https://learn.mechanic.dev/platform/liquid/filters#json-parse_json-parse_jsonl) filter. Sample JSON is easy to generate using [Shopify's GraphiQL app](https://shopify-graphiql-app.shopifycloud.com/).

{% tabs %}
{% tab title="GraphQL with stub data" %}

```liquid
{% capture query %}
  query {
    publications(first: 250) {
      edges {
        node {
          id
          name
        }
      }
    }
  }
{% endcapture %}

{% assign result = query | shopify %}

{% if event.preview %}
  {% capture result_json %}
    {
      "data": {
        "publications": {
          "edges": [
            {
              "node": {
                "id": "gid://shopify/Publication/69217648807",
                "name": "Online Store"
              }
            }
          ]
        }
      }
    }
  {% endcapture %}

  {% assign result = result_json | parse_json %}
{% endif %}

{% log available_publications: result.data.publications %}
```

{% endtab %}

{% tab title="GraphQL pagination with stub data" %}

```liquid
{% assign cursor = nil %}
{% assign total_inventory = 0 %}

{% for n in (0..100) %}
  {% capture query %}
    query {
      orders(
        first: 250
        query: "status:open"
        after: {{ cursor | json }}
      ) {
        pageInfo { hasNextPage }
        edges {
          node { name, email }
        }
      }
    }
  {% endcapture %}

  {% assign result = query | shopify %}

  {% if event.preview %}
    {% capture result_json %}
      {
        "data": {
          "orders": {
            "pageInfo": {
              "hasNextPage": false
            },
            "edges": [
              {
                "node": {
                  "name": "#1135",
                  "email": "isaac@example.com"
                }
              }
            ]
          }
        }
      }
    {% endcapture %}

    {% assign result = result_json | parse_json %}
  {% endif %}

  {% for order_edge in result.data.orders.edges %}
    {% assign order_node = order_edge.node %}

    {% if order_node.email == blank %}
      {% continue %}
    {% endif %}

    {% action "email" %}
      {
        "to": {{ order_node.email | json }},
        "subject": {{ "We're still working on " | append: order_node.name | json }},
        "body": "Thanks for your patience!"
      }
    {% endaction %}
  {% endfor %}

  {% if result.data.orders.pageInfo.hasNextPage %}
    {% assign cursor = result.data.orders.edges.last.cursor %}
  {% else %}
    {% break %}
  {% endif %}
{% endfor %}
```

{% endtab %}
{% endtabs %}