Maximizing code reusability with DRY LookML: Define fields once, use substitution operators everywhere

LookML field definitions can change over time. For example, a database column name may change, or you may need to change the definition of a LookML field for other reasons.

If you use the syntax {TABLE}.field_name to reference a database column directly in multiple places, you must update each reference manually. Any fields that reference that dimension can break if you forget to update them, and Looker will display an error:

Looker displays the error Unknown column order_items.sale_price in field list.

To make your LookML projects more efficient and easier to maintain, you can define fields in one place and use the substitution operator ($) to reference those fields everywhere else.

This page provides an example of using the substitution operator (with the syntax ${field_name}) to reference a single dimension in the definitions of multiple LookML fields.



Example: Referencing the underlying database column only once for a dimension

Define a database table column in a LookML project once using the syntax ${TABLE}.field_name in the dimension's sql parameter. Then reference the dimension using the syntax ${field_name} or ${view_name.field_name} elsewhere in your project. This lets you maintain the LookML definition of the database column in one place (the original ${TABLE}.field_name dimension), which is helpful if you need to reference it in multiple places in your project.

As an example, you can use the syntax ${TABLE}.sale_price to define a base dimension called sale_price in a view called order_items:

  dimension: sale_price {
    type: number
    value_format_name: usd
    sql: ${TABLE}.sale_price ;;
    description: "The price at which an item is set to sell."

When you define other fields that reference the sale_price dimension, can use the syntax ${sale_price} within the order_items view (or use the syntax ${order_items.sale_price} to reference the sale_price dimension in other views).

dimension: profit {
  type: number
  value_format_name: usd
  sql: ${sale_price} - ${inventory_items.cost} ;;
  description: "The difference between an item's sale price and an item's cost."

dimension: item_gross_margin {
  type: number
  value_format_name: percent_2
  sql: 1.0 * ${profit}/NULLIF(${sale_price},0) ;;

measure: total_sale_price {
  type: sum
  value_format_name: usd
  sql: ${sale_price} ;;

In this example, if the column name for the dimension sale_price changes, you will only need to update the ${TABLE}.sale_price reference once, in the definition of the base sale_price dimension. This change will then propagate automatically to the profit, item_gross_margin, and total_sale_price fields, as well as all other fields that reference the sale_price dimension.