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:
stateDiagram-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 byB
-
B
was inspired byA
-
A
has no recorded inspiration -
C
andD
were inspired byM
-
F
andE
were inspired byD
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?
Posted by Henning Koch to makandra Curriculum (2015-09-04 15:42)