The problem
I have two schemas, coffees and orders wherein I want to get the recent completed order from each coffees
First solution
defmodule Schemas.Coffee do
# ... more code
has_many :completed_orders, Schemas.Order
def coffee_query(coffee_id) do
from c in __MODULE__,
where: c.id == coffee_id,
preload: [completed_orders: Orders.completed_order],
limit: 1
end
end
defmodule Schemas.Order do
# ... more code
@completed_status :completed
def completed_order_query() do
from o in __MODULE__,
where: o.status in ^@completed_status,
order_by: [desc: o.updated_at]
end
end
defmodule Contexts.Coffees do
# more code
def get_recent_completed_coffee_order(coffee_id) do
coffee = coffee_id
|> Schemas.Coffee.coffee_query()
|> Repo.one
List.first(coffee.completed_orders)
end
end
Based from the setup, I can actually get the
completed_ordersof coffees but the only downside for me is that the preload will return alistSince it will return a
list, that will result into another process for me just to get the recent completed order.
Then, has_one/3 came into my life
defmodule Schemas.Coffee do
# ... more code
has_one :completed_order, Schemas.Order,
where: [status: :completed],
preload_order: [desc: :updated_at]
def coffee_query(coffee_id) do
from c in __MODULE__,
where: c.id == ^coffee_id
preload: [:completed_order],
limit: 1
end
end
defmodule Contexts.Coffees do
# more code
def get_recent_completed_coffee_order(coffee_id) do
coffee_id
|> Schemas.Coffee.coffee_query()
|> Repo.one
|> Map.get(:completed_order, nil)
end
end
What's the magic behind this scheme?
How does has_one/3 works?
Indicates a one-to-one association with another schema
From the current situation, coffee belongs_to order. As a result, coffee has_many orders, but I only need one result to return coming from the orders schema to identify if the coffee order is completed already.
Using has\_one/3\ I was able to achieve what I want.
has_one/3- indicates a one-to-one association with another schema.where- indicates aFiltering associationsto get my specific items.preload_order- Sets the defaultorder_byof the association, and since I am usinghas_one/3Ecto set a queryLIMITset toone.
[see has_one/3 on docs](https://hexdocs.pm/ecto/Ecto.Schema.html#has_one/3)
Happy Coding
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