Unique constraint error on a schema less many to many association table

I would set up an has many - through relation ship instead

post has_many post_tags
post has_many tags through
(Ecto.Schema — Ecto v3.14.0)

Then you can have a changeset on PostTag that validates unique constraint

I wrote a short tutorial about how I prefer to design this:

In this tutorial, can we link multiple tags with a product at once? also, by adding cast_assoc in the taggings Changeset, it will try to create a new tag every time we try to link a tag with a product, right?

There is no batch insert of tags.

I see now that I forgot to add the unique constraint in Tag-module.

  def changeset(tag, attrs) do
    tag
    |> cast(attrs, [:name])
    |> validate_required([:name])
    |> unique_constraint(:name, name: :tags_name_index)
  end

That will result in an error Changeset that you need to chose how handle (ignore or validation error).

Specs look like this:

defmodule Tutorial.TaggableTest do
  use Tutorial.DataCase

  alias Tutorial.Taggable
  alias Tutorial.Taggable.Tag
  alias Tutorial.Taggable.Tagging
  alias Tutorial.Repo
  alias Tutorial.Products

  def product_fixture(attrs \\ %{}) do
    {:ok, product} =
      attrs
      |> Enum.into(%{description: "some description", name: "some name", price: 120.5, properties: %{}})
      |> Products.create_product()

    product
  end

  def tag_fixture() do
    product = product_fixture()

    {:ok, %Tagging{tag: tag}} =
      Taggable.tag_product(product, %{tag: %{name: "Stout"}})

    tag
  end

  describe "tags and taggings" do
    test "list_tags/0 returns all tags" do
      tag = tag_fixture()
      assert Taggable.list_tags() == [tag]
    end

    test "tag_product/2 with valid data creates a tag" do
      product = product_fixture()
      assert {:ok, %Tagging{} = tagging} = Taggable.tag_product(product, %{tag: %{name: "Stout"}})
      assert tagging.tag.name == "Stout"
    end

    test "tag_product/2 with invalid data returns error changeset" do
      product = product_fixture()
      assert {:error, %Ecto.Changeset{}} = Taggable.tag_product(product, %{tag: %{name: nil}})
    end

    test "tag_product/2 with duplicate tag returns error changeset" do
      product = product_fixture()
      Taggable.tag_product(product, %{tag: %{name: "Stout"}})
      assert {:error, %Ecto.Changeset{}} = Taggable.tag_product(product, %{tag: %{name: "Stout"}})
    end

    test "delete_tag_from_product/2 deletes the tagging from product but not the tag" do
      product = product_fixture()
      {:ok, %Tagging{tag: %Tag{} = tag}} = Taggable.tag_product(product, %{tag: %{name: "Lager"}})

      assert %{tags: [^tag]} = product |> Repo.preload(:tags)
      assert {:ok, %Tagging{}} = Taggable.delete_tag_from_product(product, tag)
      assert %{tags: []} = product |> Repo.preload(:tags)
      assert [%Tag{name: "Lager"}] = Taggable.list_tags()
    end
  end
end

Then how do you create product with multiple tags?

I would loop through the tags and insert them one by one (can’t be that many). Otherwise I risk getting an error if I batch insert and one of them is a duplicate.

this is why I tried doing.

And why did you use that cast_assoc in the taggings Changeset?

Its actually through the Tagging I insert the tag.

  def tag_product(product, attrs \\ %{}) do
    product
    |> Ecto.build_assoc(:taggings)
    |> Tagging.changeset(attrs)
    |> Repo.insert()
  end

But I think there is an issue that only allows me to create and assign new tags. Will see if I can fix that.

exactly… so every time I tag something, it tries to create a new one, and that causes the unique constraint in tag table to raise

also, i keep getting this error,

* (Ecto.ConstraintError) constraint error when attempting to insert struct:

    * taggings_product_id_fkey (foreign_key_constraint)

Why is that?

I am using Ecto.Multi to create the Product and then associate it with the tags