Follows in common

On Twitter and many other social networks there's a useful feature of showing you which follows you have in common with another user. In this example we'll show how to implement this as a simple Datalog query.

note

You can play along inside your browser's console!

First we'll create the tables that hold our data. Notice that the data is not nested. This is a feature! Data is laid out flat, or normalized. This gives us greater flexibility in querying data and prevents us from having to duplicate information across different parts of a deeply nested data structure since each row in a table only holds the data that it cares about.

// Already in the browser's scope
// import * as datalog from '@datalogui/datalog'

// Not actually needed for this example
const Users = datalog.intoTable([
  { username: "daiyi" },
  { username: "marco" },
  { username: "noah" },
  { username: "paul" }
  // ...
])

const Follows = datalog.intoTable([
  { username: "marco", follows: "daiyi" },
  { username: "noah",  follows: "daiyi" },
  { username: "marco", follows: "paul" },
  { username: "noah",  follows: "paul" },
  { username: "marco", follows: "noah" },
  { username: "noah",  follows: "marco" },
  { username: "paul",  follows: "noah" },
  // ...
])

Simple enough. We have a table that defines the Users in this network, and another table that defines who follows whom. Let's write a query that will ask for myself ("marco") and "noah", who do we both follow?

const Query = datalog.query(({commonFollow}) => {
  Follows({username: "marco", follows: commonFollow})
  Follows({username: "noah", follows: commonFollow})
})

// The Query variable represents the query, but if we want to see that data we
// need to use the `.view()` method. The follow `.readAllData()` call returns
// all the data from the query instead of just the recent results from any
// recent changes.
Query.view().readAllData()

// => [{"commonFollow":"daiyi"},{"commonFollow":"paul"}]

4 lines of code for a common followers query! not bad.