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
Mwas inspired by
Bwas inspired by
Ahas no recorded inspiration
Dwere inspired by
Ewere inspired by
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?
- Read about different patterns for storing trees in databases.
- Migrate from
Movie#inspiration_idto 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
ancestryis triggering. How does it work?