CommonChanges is a collection of functions to help with managing and creating our &changeset/2 function in our schemas.

Preloading associations on change

Often times we want to be able to change an association with (put/cast)_assoc, but we have an awkwardness of having to use a preload in a spot to do this. We can aleviate that by doing the following:

defmodule MyApp.Accounts.User do
  def changeset(changeset, params) do
    changeset
      |> cast([:name, :email])
      |> validate_required([:name, :email])
      |> EctoShorts.CommonChanges.preload_change_assoc(:address)
  end
endcopy

Doing this allows us to then pass address in via a map, or even using the struct from the database directly to add as a relation

Validating relation is passed in somehow

We can validate for a relation being passed in via id or by using our preload_change_assoc by doing the following:

defmodule MyApp.Accounts.User do
  def changeset(changeset, params) do
    changeset
      |> cast([:name, :email, :address_id])
      |> validate_required([:name, :email])
      |> EctoShorts.CommonChanges.preload_change_assoc(:address,
        required_when_missing: :address_id
      )
  end
endcopy

Conditional functions

We can also run functions when something happens by defining conditional functions like so:

defmodule MyApp.Accounts.User do
  alias EctoShorts.CommonChanges

  def changeset(changeset, params) do
    changeset
      |> cast([:name, :email, :address_id])
      |> validate_required([:name, :email])
      |> CommonChanges.put_when(
        &CommonChanges.changeset_field_nil?(&1, :email),
        &put_change(&1, :email, "some_default@gmail.com")
      )
  end
endcopy