Computed Fields

A computed field is a field in an entity that is not in the data source, but is the result of processing using the value of other fields. A computed field is then ignored in each insert or edit operation and is automatically populated by the system in each read operation where site is requested.

Typetta offers convenient middleware that can be used to define calculated fields. The following example shows how you can define a fullName field that is the composition of firstName and lastName of a hypothetical entity that represents a user. These last two fields are actually historicised in the data source, while fullName is in fact a virtual field.

type User @entity @mongo {
  id: ID! @id
  firstName: String!
  lastName: String!
  fullName: String! @default(from: "middleware")
}

const entityManager = new EntityManager({
  overrides: {
    user: {
      middlewares: [
        computedField({
          fieldsProjection: {
            fullName: true
          },
          requiredProjection: {
            firstName: true,
            lastName: true
          },
          compute: async (user) => {
            return {
              fullName: `${user.firstName} ${user.lastName}`,
            }
          }
        })
      ],
    },
  }
}

In the previous example, fieldsProjection represents the set of computed fields, requiredProjection represents the set of fields that are required for processing, and it computes the function that receives the fields on the data source as input and must provide the computed fields downstream of the processing as output.

Note that this middleware uses the Projection Dependency Middleware described above with a composition and reuse logic.