Check out example codes for "rails erd". It will help you in understanding the concepts better.

Code Example 1

attributes:
  - content
  - foreign_key
  - inheritance
disconnected: true
filename: erd
filetype: pdf
indirect: true
inheritance: false
markup: true
notation: simple
orientation: horizontal
polymorphism: false
sort: true
warn: true
title: sample title
exclude: null
only: null
only_recursion_depth: null
prepend_primary: false
cluster: false
splines: spline

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