How to write a RethinkDB merge query which handles null results

Question

How can I write a RethinkDB query for a document which returns a merged version of the document if it is found, and null otherwise?

The following, naive, query (written in JavaScript) causes an exception to be thrown from .merge in case .get returns null (ie the user isn't found):

r.table('users')
  .get(username)
  .merge((user) => {
    return {
      'projects': r.table('projects').getAll(username, {index: 'owner',})
        .coerceTo('array'),
    }
  })
  .run(conn)

Answer 1

Writing a merge query, while handling the case where a document is not found, can be accomplished through the use of do and branch , so that merge only gets invoked on non-null results:

r.table('users')
  .get(username)
  .do((user) => {
    // Only perform a merge if user isn't null
    return r.branch(
      user.eq(null),
      null,
      user.merge({
        'projects': r.table('projects').getAll(username, {index: 'owner',})
          .coerceTo('array'),
      })
    )
  })
  .run(conn)

Answer 2

您可以在合并查询之后添加.default(null)来处理不存在的错误（尽管这也将处理合并查询中的任何其他不存在的错误）。