mastodon/app/models/follow_suggestion.rb

class FollowSuggestion
  def self.get(for_account_id, limit = 6)
    neo = Neography::Rest.new

    query = <<END
START a=node:account_index(Account={id})
MATCH (a)-[:follows]->(b)-[:follows]->(c)
WHERE a <> c
AND NOT (a)-[:follows]->(c)
RETURN DISTINCT c.account_id, count(b), c.nodeRank
ORDER BY count(b) DESC, c.nodeRank DESC
LIMIT {limit}
END

    results = neo.execute_query(query, id: for_account_id, limit: limit)

    if results.empty?
      results = fallback(for_account_id, limit)
    elsif results['data'].size < limit
      results['data'] = (results['data'] + fallback(for_account_id, limit - results['data'].size)['data']).uniq
    end

    account_ids  = results['data'].map(&:first)
    blocked_ids  = Block.where(account_id: for_account_id).pluck(:target_account_id)
    accounts_map = Account.where(id: account_ids - blocked_ids).with_counters.map { |a| [a.id, a] }.to_h

    account_ids.map { |id| accounts_map[id] }.compact
  rescue Neography::NeographyError, Excon::Error::Socket => e
    Rails.logger.error e
    return []
  end

  private

  def self.fallback(for_account_id, limit)
    neo = Neography::Rest.new

    query = <<END
START a=node:account_index(Account={id})
MATCH (b)
WHERE a <> b
AND NOT (a)-[:follows]->(b)
RETURN b.account_id
ORDER BY b.nodeRank DESC
LIMIT {limit}
END

    neo.execute_query(query, id: for_account_id, limit: limit)
  end
end