Cast_assoc update on unique index conflict

Questions / Help
#1 
  schema "votes" do
    belongs_to :entry, Entry
    belongs_to :account, Account

    timestamps(type: :utc_datetime)
  end

  schema "accounts" do
    field :email, :string
    field :password, :string, virtual: true
    field :encrypted_password, :string

    has_many :votes, Vote

    timestamps(type: :utc_datetime)
  end

  schema "entries" do
    field :author, :string
    field :author_id, :integer
    field :content, :string
    field :entry_id, :integer
    field :entry_timestamp, :string
    field :favorite_count, :integer
    field :topic_uri, :string

    has_many :votes, Vote

    timestamps(type: :utc_datetime)
  end

I have the three schemas above and I am trying to create a Vote using cast_assoc and put_assoc.

Below is a changeset function for a Vote. The problem is I’ve got a unique index on entry_id (it’s not the primary key) so when I pass the entry_id if an entry with that id exists than cast_assoc throws unique constraint validation error.

How can I make it so that cast_assoc actually updates as well on entry_id or find by entry_id maybe?

  def changeset(vote, %{account: account, entry: entry} = attrs) do
    vote
    |> cast(attrs, [])
    |> cast_assoc(:entry, with: &DertGG.Entries.Entry.changeset/2)
    |> put_assoc(:account, account)
  end