Understanding References

Read the article below or view a tutorial walk-through in the AppSheet Editor:

Introduction

Once you have an app with multiple tables, it's often useful to create connections, known as references, between the app's tables.

For example, an order capture app might contain the following tables:

  1. Orders, with one row per order.
  2. Order Details, with one row per line item.
  3. Products, with one row per product being sold.
  4. Customers, with one row per customer.

The order capture app might contain the following references:

  1. Each Orders row will reference the Customers row of the customer who placed that order.
  2. Each Order Details row will reference its parent Orders row.
  3. Each Order Details row will reference its corresponding Products row.

References serve three purposes:

  1. They allow you to represent relationships. For example, the reference between the Orders row and the Customers row allows you to capture the relationship between an order and the customer who placed that order.
  2. They allow you to easily retrieve information from a related row in another table. For example, the reference between the Order Details row and the Products table allows you to start from an individual Order Details row and retrieve the name, image, and price column values from the related Products row.
  3. They allow you to navigate from one row to another. For example, you might retrieve a particular Order Detail row then navigate to the related Orders rows, and then to the related Customers rows.

Adding a Reference

You create a reference by adding a column of type Ref to a table and specifying the related table's name. For example, in the Orders table you would add a Ref to the Customers table. In the Order Details table you would add one Ref to the Orders table and another to the Products table.

Reverse References

For each Ref you add, the system automatically adds a reverse reference in the opposite direction. For example, when you add the Ref from the Orders table to the Customers table, the system automatically adds a reverse reference from the Customers table to the Orders table.

Reverse references serve three purposes:

  1. They allow you to navigate from one row to all of its related rows.
  2. They allow the user interface to easily display a row along with all of its related rows in another table.
  3. They allow aggregates to be computed like the count of a customer's orders or the total dollar value of a customer's orders.

References and Reverse References Contain Key Values

A Ref column always stores the key column value of the referenced row. For example, if the key column value of a Customers row is Ann Adams then the Ref field in the related Orders row will contain the value Ann Adams. A table's key column value uniquely identifies each row in that table. The copy of the key column value in the Ref column allows the system to unambiguously retrieve the proper row in the referenced table. You can read more about key columns here.

The system-added reverse reference column is a list of the key column values of the related rows. For example, the reverse reference in Customers record Ann Adams would contain 1003, 1005, and 1010 if those were the key column values of Ann's related Orders records. When viewing an individual row, related reverse references appear as inline views that can be customized.

Expressing Ownership Between Tables

References can indicate not only that two tables are related, but that rows of one table should be owned by (or considered a part of) rows from another table. This is done by activating the IsAPartOf option in the Ref column structure. Typically, this should only be done in cases where rows containing the Ref column only make sense when associated with a row from the referenced table. For example, you may have a separate Order Details table for line items that reference an Order, but conceptually each entry should be considered "part of" an Order and shouldn't exist independently (put another way, the Order record "owns" the Order Details that reference it).

There are several implications of enabling this ownership relationship:

  1. Form views should allow users to add or update related rows that are a part of that row without leaving the form.
  2. A form comprised of multiple rows should be treated as a single update.
  3. If a row is deleted, any related rows that are a part of it should also be deleted (and again treated as a single update).

The first of these to take effect will be #1: System-generated reverse reference columns related to Ref columns marked IsAPartOf will appear in form views and allow users to view, add, and edit line items within the form view. The availability of these options is limited by the table or slice permissions (and until the final form is saved, pending adds can still be modified even if the table doesn't allow updates).

There are some limitations of which to be aware:

  • A table can only have one Ref column marked IsAPartOf (a row can only be a part of one other row).
  • Actions are not supported within forms. Inline views based on reverse references in forms will not display actions or trigger actions based on view events.
  • Canceling a form with pending line item adds/edits also cancels those adds/edits.

Creating References

A reference can be added to a table in two ways.

  • AppSheet automatically adds references when you generate or regenerate a table.
  • You can manually add references.

System-Added References

When you initially create your app or when you regenerate a table's column structure, AppSheet will try to automatically infer references between tables. For example, if you have a Customers table with a Name column as its key, and if the Orders table has a column called Customer Name, the Customer Name column is assumed to be a Ref column.

User-Added References

You can add a reference as follows:

  1. Ensure your worksheet has a column to contain the reference.
  2. If you needed to add a new worksheet column to contain the reference, go to the Data >> Column Structure pane and click the Regenerate Structure button for the table. This will include the newly added worksheet column in the table.
  3. Go to the Data >> Columns pane, show the table schema, and select the column you wish to make a reference.
  4. In the Type dropdown, select Ref.
  5. In the Source table dropdown, select the referenced table.
  6. Click Done.
  7. Save the changes.

System-Added Reverse References

Whenever a Ref column is created, AppSheet automatically creates a "reverse reference" virtual column in the referenced table. This is true whether you add the reference or the system does.

The reverse reference virtual column is given a default name that you can change.

The reverse reference virtual column is assigned an app formula that uses the special function REF_ROWS(). Behind the scenes, REF_ROWS() performs a complex SELECT() operation.

For both references and reverse references, reference icons are displayed.

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