Building a JSON map for a self-referencing Ecto model - json

Question

I have an Ecto model as such:
defmodule Project.Category do
use Project.Web, :model
schema "categories" do
field :name, :string
field :list_order, :integer
field :parent_id, :integer
belongs_to :menu, Project.Menu
has_many :subcategories, Project.Category, foreign_key: :parent_id
timestamps
end
#required_fields ~w(name list_order)
#optional_fields ~w(menu_id parent_id)
def changeset(model, params \\ :empty) do
model
|> cast(params, #required_fields, #optional_fields)
end
end
As you can see the Category model can reference itself via the subcategories atom.
Here is the view associated with this model:
defmodule Project.CategoryView do
use Project.Web, :view
def render("show.json", %{category: category}) do
json = %{
id: category.id,
name: category.name,
list_order: category.list_order
parent_id: category.parent_id
}
if is_list(category.subcategories) do
children = render_many(category.subcategories, Project.CategoryView, "show.json")
Map.put(json, :subcategories, children)
else
json
end
end
end
I have an if condition on subcategories so that I can play nice with Poison when they are not preloaded.
Finally, here are my 2 controller functions that invoke this view:
defmodule Project.CategoryController do
use Project.Web, :controller
alias Project.Category
def show(conn, %{"id" => id}) do
category = Repo.get!(Category, id)
render conn, "show.json", category: category
end
def showWithChildren(conn, %{"id" => id}) do
category = Repo.get!(Category, id)
|> Repo.preload [:subcategories, subcategories: :subcategories]
render conn, "show.json", category: category
end
end
The show function works fine:
{
"parent_id": null,
"name": "a",
"list_order": 4,
"id": 7
}
However, my showWithChildren function is limited to 2 levels of nesting because of how I use preloading:
{
"subcategories": [
{
"subcategories": [
{
"parent_id": 10,
"name": "d",
"list_order": 4,
"id": 11
}
],
"parent_id": 7,
"name": "c",
"list_order": 4,
"id": 10
},
{
"subcategories": [],
"parent_id": 7,
"name": "b",
"list_order": 9,
"id": 13
}
],
"parent_id": null,
"name": "a",
"list_order": 4,
"id": 7
}
For example, the category item 11 above also has subcategories but I am unable to reach them. Those subcategories can also have subcategories themselves, so the potential depth of the hierarchy is n.
I am aware that I need some recursive magic but since I'm new to both functional programming and Elixir, I cannot wrap my head around it. Any help is greatly appreciated.

Answer 1

You can consider doing the preloading in the view, so it works recursively:
def render("show.json", %{category: category}) do
%{id: category.id,
name: category.name,
list_order: category.list_order
parent_id: category.parent_id}
|> add_subcategories(category)
end
defp add_subcategories(json, %{subcategories: subcategories}) when is_list(subcategories) do
children =
subcategories
|> Repo.preload(:subcategories)
|> render_many(Project.CategoryView, "show.json")
Map.put(json, :subcategories, children)
end
defp add_subcategories(json, _category) do
json
end
Keep in mind this is not ideal for two reasons:
Ideally you don't want to do queries in views (but this is is recursive, so it is easier to piggyback in the view rendering)
You are going to emit multiple queries for the second level of subcategories
There is a book called SQL Antipatterns and, if I am not mistaken, it covers how to write tree structures. Your example is exposed as an antipattern in one of the free chapters. It is an excellent book and they explore solutions for all antipatterns.
PS: you want show_with_children and not showWithChildren.