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:
- Customers table having one record per customer.
- Products table having one record per product being sold.
- Orders table having one record per order taken.
- Order Details table having one record per line item in its parent order.
The order capture app might contain the following references:
- Each Orders record will reference the Customers record of the customer who placed that order.
- Each Order Details record will reference its parent Orders record.
- Each Order Details record will reference its corresponding Products record.
References serve three purposes:
- They allow you to represent relationships. For example, the reference between the Orders record and the Customers record allows you to capture the relationship between an order and the customer who placed that order.
- They allow you to easily retrieve information from a related record in another table. For example, the reference between the Order Details record and the Products table allows you to start from an individual Order Details record and retrieve the name, image, and price fields from the related Products record.
- They allow you to navigate from one record to another. For example, you might retrieve a particular Order Detail record then navigate to the related Orders records, and then to the related Customers records.
Adding a Reference
You create a reference by adding a Ref type field 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.
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:
- They allow you to navigate from one record to all of its related records.
- They allow the user interface to easily display a record along with all of its related records in another table.
- 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 type field always stores the key value of the referenced record. For example, if the key value of a Customers record is "Ann Adams" then the Ref field in the related Orders record will contain the value "Ann Adams". A table's key value uniquely identifies each record in that table. The copy of the key value in the Ref field allows the system to unambiguously retrieve the proper record in the referenced table. You can read more about key columns here.
The system-added reverse reference field is a list of the key values of the related records. For example, the reverse reference in Customers record "Ann Adams" would contain "1003", "1005", and "1010" if those were the key values of Ann's related Orders records. When viewing an individual records, 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 records of one table should be owned by (considered a part of) records 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 records containing the Ref column only make sense when associated with a record 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:
- Form views should allow users to add or update related records that are a part of that record without leaving the form.
- A form comprised of multiple records should be treated as a single update.
- If a record is deleted, any related records that are a part of it should also be deleted (and again treated as a single update).
These behaviors haven't yet been fully implemented, but all three should be considered when deciding whether to activate this option.
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 record can only be a part of one other record).
- 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.
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.
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" field is assumed to be a Ref column.
You can add a reference as follows:
- Ensure your worksheet has a column to contain the reference.
- If you needed to add a new worksheet column to contain the reference, go to the Data > Column Structure pane and click the Regenerate button for the table. This will include the newly added worksheet column in the table.
- Go to the Data > Column Structure pane, show the table schema, and select the column you wish to make a reference.
- In the "Column Type" dropdown, select "Ref".
- In the "ReferencedTableName" dropdown, select the referenced table.
- Click "OK".
- 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(). REF_ROWS() is the name of a complex SELECT() function.
For both references and reverse references, reference icons are displayed.