Insert multiple rows at once with Ecto. "protocol Enumerable not implemented for #Ecto.Changeset"

Question

I have two tables. A table of topics which has_many tweets. My table of tweets belongs_to a topic.

Topic Schema:

defmodule Sentiment.Topic do
  use Sentiment.Web, :model

  schema "topics" do
    field :title, :string

    has_many :tweets, Sentiment.Tweet
  end

  def changeset(struct, params \\ %{}) do
    struct
    |> cast(params, [:title])
    |> validate_required([:title])
  end
end

Tweet Schema:

defmodule Sentiment.Tweet do
  use Sentiment.Web, :model

  schema "tweets" do
    field :message, :string
    belongs_to :topic, Sentiment.Topic

  end

  @doc """
  Builds a changeset based on the `struct` and `params`.
  """
  def changeset(struct, params \\ %{}) do
    struct
    |> cast(params, [:message])
    |> validate_required([:message])
  end
end

I am attempting to insert a topic into my table, followed by 500 tweets after I run a twitter search for that topic.

In my controller, I use Ecto.Multi to group my repo operations, however, each time I run my operation I get an error protocol Enumerable not implemented for #Ecto.Changeset<action: nil, changes: %{message: "\"aloh....

This is how I am attempting to insert my topic first, obtain it's id, and then insert a tweet message with the associated id with one transaction.

 def create(conn, %{"topic" => topic}) do
    # create a topic changeset
    topic_changeset = Topic.changeset(%Topic{}, topic)

    # obtain a list of tweet messages: ["hello", "a tweet", "sup!"]
    %{"title" => title} = topic
    all_tweets = title
    |> Twitter.search

# create an Ecto.Multi struct.
multi =
  Ecto.Multi.new
  |> Ecto.Multi.insert(:topics, topic_changeset) #insert topic
  |> Ecto.Multi.run(:tweets, fn %{topics: topic} ->
    changeset_tweets = all_tweets
    |> Enum.map(fn(tweet) ->
      %{topic_id: topic.id, message: tweet}
    end)

    Repo.insert_all(Tweet, changeset_tweets)

  end)

      # Run the transaction
      case Repo.transaction(multi) do # ERROR HERE!
        {:ok, result} ->
          conn
          |> put_flash(:info, "Success!")
          |> redirect(to: topic_path(conn, :index))
        {:error, :topics, topic_changeset, %{}} ->
          conn
          |> put_flash(:error, "Uh oh...")
          |> render("new.html", changeset: topic_changeset)
        {:error, :tweets, topic_changeset, %{}} ->
          conn
          |> put_flash(:error, "Something really bad happened...")
          |>render("new.html", changeset: topic_changeset)
      end
  end

How can I insert_all about 500 rows in one transaction using Ecto.Multi?

Update I have converted the list of changesets into a list of maps and my error has changed to something even more confusing.

Answer 1

For Ecto.Multi to properly progress with steps, every one of them has to return either {:ok, value} or {:error, reason} tuple.

When inserting, updateing or deleteing a changeset, it will return such a tuple automatically, but for run, you need to return it explicitly.

Please, consider the following:

Ecto.Multi.new
|> Ecto.Multi.insert(:topics, topic_changeset) #insert topic
|> Ecto.Multi.run(:tweets, fn %{topics: topic} ->
   maps = 
     Enum.map(all_tweets, fn(tweet) ->
       %{topic_id: topic.id, message: tweet}
     end)

    {count, _} = Repo.insert_all(Tweet, maps)
    {:ok, count} # <----
end)

Answer 2

Alex, this is not a direct answer to your question with Ecto.Multi, but a suggestion, that it might be easier to use cast_assoc(:tweets) inside your topic changeset.

This would look like this:

# Topic.ex
...
def changeset(struct, params \\ %{}) do
  struct
  |> cast(params, [:message])
  |> cast_assoc(:tweets) 
  |> validate_required([:message])
end

# create_topic..
...
tweets = [%{message: "Tweet 1"}, %{message: "Tweet 2"}]

{:ok, topic} = 
  %Topic{}
  |> Topic.changeset(Map.put(topic, :tweets, tweets))
  |> Repo.insert()

Answer 3

Note that as of now (2021) Ecto 3 provides the function Multi.insert_all already, so that one wouldn't need to use Multi.run manually in this scenario.