Posted almost 5 years ago. Visible to the public.

Persisting trees [2d]

Storing a tree

For each movie in MovieDB, we want to track which other movie it was inspired by. For example:

  • "Interstellar was inspired by 2001"
  • "Inception was inspired by The Matrix"

Start by adding a field Movie#inspiration_id. In the movie form, I should be able to select the inspirating movie.

The list should:

  • include all the movies that were released in previous years
  • not include the movie itself
  • not include a movie that was itself inspired by the movie that is being edited.


In the movie show view, I want to see the "inspiration tree" of that movie.

An inspiration tree for a movie M could look like this:

A | B | (M) / \ C D / \ F E

In the tree above I can see:

  • the current movie M was inspired by B
  • B was inspired by A
  • A has no recorded inspiration
  • C and D were inspired by M
  • F and E were inspired by D

Note that if an ancestor B has inspired other movies than the current movie M, we do not need to display it.

Look at your development log while opening a movie. How many queries are triggered?

Materialized path

  • Read about different patterns for storing trees in databases.
  • Migrate from Movie#inspiration_id to the materialized path pattern using the ancestry gem.
  • Make sure that you write a proper migration for existing movies.
  • Reduce the number of queries for the inspiration tree view to 2.
  • Look at the queries that ancestry is triggering. How does it work?

Owner of this card:

Henning Koch
Last edit:
16 days ago
by Henning Koch
Posted by Henning Koch to makandra Curriculum
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