Adding The Favorite Flag

User Favorites

One way that we could find the user’s favorites would be to run a separate MATCH clause within the same query. But as all of the routes that interact with the MovieService are used by both anonymous and logged in users, this could add unwanted complexity to the service.

Instead, it would be cleaner to execute two separate queries within the same transaction; one query to fetch a list of the user’s favorites, and another to retrieve a list of movies.

Fortunately, with only a few minor tweaks to the code, we can create a method that can be used to populate the favorite flag in the every other method in the MovieService.

Finding Favorites

Modify the getUserFavorites() method to run the following query against the tx parameter.

MATCH (u:User {userId: $userId})-[:HAS_FAVORITE]->(m)
RETURN m.tmdbId AS id

async getUserFavorites(tx, userId) {
  // If userId is not defined, return an empty array
  if ( userId === undefined ) {
    return []
  }

  const favoriteResult = await tx.run(
    `
      MATCH (:User {userId: $userId})-[:HAS_FAVORITE]->(m)
      RETURN m.tmdbId AS id
    `,
    { userId, }
  )

  // Extract the `id` value returned by the cypher query
  return favoriteResult.records.map(
    row => row.get('id')
  )
}

Multiple Transaction Calls

This part of the challenge requires you to run two queries within the same transaction,

The all() method currently uses a function to immediately return the results of tx.run().

Instead of returning the results directly, you’ll need to update the query to be an async function which await s the the results of a call to getUserFavorites(tx, userId), and then passes the results as a parameter to the existing query.

async getUserFavorites(tx, userId) {
  // If userId is not defined, return an empty array
  if ( userId === undefined ) {
    return []
  }

  const favoriteResult = await tx.run(
    `
      MATCH (:User {userId: $userId})-[:HAS_FAVORITE]->(m)
      RETURN m.tmdbId AS id
    `,
    { userId, }
  )

  // Extract the `id` value returned by the cypher query
  return favoriteResult.records.map(
    row => row.get('id')
  )
}

Comparing Versions

If we take a look at the two versions of the all() method, not much has changed. The favorites array has been passed through as a parameter to the query, and the query now uses the Cypher IN clause to check if the ID is included in the array.

// Execute a query in a new Read Transaction
const res = await session.executeRead(
  async tx => {
    const favorites = await this.getUserFavorites(tx, userId)

    return tx.run(
      `
        MATCH (m:Movie)
        WHERE m.\`${sort}\` IS NOT NULL
        RETURN m {
          .*,
          favorite: m.tmdbId IN $favorites
        } AS movie
        ORDER BY m.\`${sort}\` ${order}
        SKIP $skip
        LIMIT $limit
      `, { skip: int(skip), limit: int(limit), favorites })
  }
)

// Execute a query in a new Read Transaction
const res = await session.executeRead(
  tx => tx.run(
    `
      MATCH (m:Movie)
      WHERE m.\`${sort}\` IS NOT NULL
      RETURN m {
        .*
      } AS movie
      ORDER BY m.\`${sort}\` ${order}
      SKIP $skip
      LIMIT $limit
    `, { skip: int(skip), limit: int(limit) })
)