How to model a many self-referential relationship with many parents?

Question

I want to model out the following in the easiest way:

A skill has many dependent skills.

Each skill should exist on their own, and a skill may have other skills

Answer 1

Instead of iterating through the tree (more like a directed graph actually) each time you need to retrieve all dependencies for a skill, you might just iterate through the implied dependencies when adding a new dependency to a particular skill and save these to a table called 'Dependency' which maps a skill to a dependency and vice versa. For example (the relations could be better worded):

class Skill
    has_many :dependers, class_name: 'Dependency', foreign_key: :dependee_id
    has_many :dependees, class_name: 'Dependency', foreign_key: :depender_id

    has_many :dependencies, through: :dependees
    has_many :depending, through: :dependers

    def add_dependency(skill)
        recurse_dependencies(skill)
    end

    def recurse_dependencies(skill)
        # perform this check to avoid circular and duplicate dependencies
        if !depender_ids.include?(skill.id) && !dependee_ids.include?(skill.id)
            dependee_ids << skill.id
        end

        skill.dependencies.each do |dependency|
            recurse_dependencies(dependency)
        end
    end
end

class Dependency
    belongs_to :dependee
    belongs_to :depender
end

You should then be able to do things like:

@front_end_development.dependencies
@front_end_development.depending
@front_end_development.add_dependency(@html)