940 Bonus: 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 inspiring 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.

Traversal

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:

CopystateDiagram-v2
    M: (M)
    A --> B
    B --> M
    M --> C
    M --> D
    D --> F
    D --> 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 Show archive.org snapshot .
• 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?