My concern with working with associations in this case is having to update the record, I came up with a solution and would like feedback on it to see if it is correct or even viable.
I’m working on a project where users have inventories, obviously in their inventories they have items, I wanted to keep track of how much of an item an user has, so my schema goes as follows:
User:
schema "users" do
...
has_one(:inventory, Inventory)
timestamps()
end
Inventory:
schema "inventories" do
field(:item_quantities, {:array, :map}, default: [])
belongs_to(:user, User)
many_to_many(:items, Item, join_through: "inventory_items")
timestamps()
end
Item:
schema "items" do
...
many_to_many(:inventories, Inventory, join_through: "inventory_items")
timestamps()
end
The solution I came up with to keep track of the amount of a item an user has is with the field(:item_quantities, {:array, :map} part. With a common module I update this value so it is less error prone when updating the values (adding, removing, etc…) These are the relevant functions on the module:
@doc """
Updates 'item_id' on 'inventory' adding 'amount' to it.
"""
def add_inventory_item_amount(%Inventory{} = inventory, item_id, amount) do
updated_items =
Enum.map(inventory.item_quantities, fn item ->
case item["id"] == item_id do
true ->
%{item | "amount" => item["amount"] + amount}
false ->
item
end
end)
inventory
|> Ecto.Changeset.change(%{item_quantities: updated_items})
|> Repo.update!()
end
@doc """
Updates 'item_id' on 'inventory' removing 'amount' from it.
"""
def remove_inventory_item_amount(%Inventory{} = inventory, item_id, amount) do
updated_items =
Enum.map(inventory.item_quantities, fn item ->
case item["id"] == item_id do
true ->
%{item | "amount" => item["amount"] - amount}
false ->
item
end
end)
inventory
|> Ecto.Changeset.change(%{item_quantities: updated_items})
|> Repo.update!()
end
@doc """
Adds 'item_amount' to 'inventory' of 'item'. Mostly used to add new items to the inventory.
"""
def add_item_to_inventory(%Item{} = item, %Inventory{} = inventory, item_amount) do
params = %{id: item.id, amount: item_amount}
inv = Repo.preload(inventory, :items)
inv
|> Ecto.Changeset.change(%{item_quantities: [params | inv.item_quantities]})
|> Ecto.Changeset.put_assoc(:items, [item | inv.items])
|> Repo.update!()
end
My question is, is this a valid way to do what I want to do or can I work directly with the associations without keeping track on a separate field on the schema?
