many_to_many relationships in Ecto and Phoenix for Products and Tags

The other day I was helping a friend set up a phoenix app which required the use of tags on products, we all have used tags in our day to day to add information about notes, images, and other stuff. Tags are just labels/chunks-of-text which are used to associated with an entity like a product, blog post, image, etc. This blog post has a few tags too (Ecto, Elixir, Phoenix, etc.). Tags help us organize information by annotating records with useful fragments of information. And modeling these in a database is pretty straightforward, it is usually implemented like the following design.

As you can see, we have a many-to-many relation between the products and tags tables via a products_tags table which has just 2 columns the product_id and the tag_id and it has a composite primary key (while also having an index on the tag_id to make lookups faster). The use of a join table is required, however, you usually want the join table to be invisible in your domain, as you don’t want to deal with a ProductTag model, it doesn’t serve any purpose other than helping you bridge the object model with the relational model. Anyway, here is how we ended up building the many-to-many relationship in Phoenix and Ecto.

Scaffolding the models

We use a nondescript Core context for our Product model by running the following scaffold code:

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mix phx.gen.html Core Product products name:string description:text

This generates the following migration (I’ve omitted the boilerplate to make reading the relevant code easier):

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create table(:products) do
add :name, :string
add :description, :text

timestamps()
end

Don’t forget to add the following to your router.ex

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resources "/products", ProductController

Then, we add the Tag in the same context by running the following scaffold generator:

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mix phx.gen.html Core Tag tags name:string:unique

This generates the following migration, note the unique index on name, as we don’t want tags with duplicate names, you might have separate tags per user in which case you would have a unique index on [:user_id, :name].

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create table(:tags) do
add :name, :string

timestamps()
end

create unique_index(:tags, [:name])

Finally, we generate the migration for the join table products_tags(by convention it uses the pluralized names of both entities joined by an underscore so products and tags joined by an _ gives us the name products_tags).

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mix phx.gen.schema Core.ProductTag products_tags product_id:references:products tag_id:references:tags

This scaffolded migration requires a few tweaks to make it look like the following:

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create table(:products_tags, primary_key: false) do
add :product_id, references(:products, on_delete: :nothing), primary_key: true
add :tag_id, references(:tags, on_delete: :nothing), primary_key: true
end

create index(:products_tags, [:product_id])
create index(:products_tags, [:tag_id])

Note the following:

  1. We added a primary_key: false declaration to the table() function call to avoid creating a wasted id column.
  2. We got rid of the timestamps() declaration as we don’t want to track inserts and updates on the joins. You might want to track inserts if you want to know when a product was tagged with a specific tag which makes things a little more complex, so, we’ll avoid it for now.
  3. We added a , primary_key: true to the :product_id and :tag_id lines to make [:product_id, :tag_id] a composite primary key

Now our database is set up nicely for our many-to-many relationship. Here is how our tables look in the database:

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product_tags_demo_dev=# \d products
Table "public.products"
┌─────────────┬────────────────────────────────┬───────────┬──────────┬─────────────────────────────┐
│ Column │ Type │ Collation │ Nullable │ Default │
├─────────────┼────────────────────────────────┼───────────┼──────────┼─────────────────────────────┤
│ id │ bigint │ │ not null │ nextval('products_id_seq'::…│
│ │ │ │ │…regclass) │
│ name │ character varying(255) │ │ │ │
│ description │ text │ │ │ │
│ inserted_at │ timestamp(0) without time zone │ │ not null │ │
│ updated_at │ timestamp(0) without time zone │ │ not null │ │
└─────────────┴────────────────────────────────┴───────────┴──────────┴─────────────────────────────┘
Indexes:
"products_pkey" PRIMARY KEY, btree (id)
Referenced by:
TABLE "products_tags" CONSTRAINT "products_tags_product_id_fkey" FOREIGN KEY (product_id) REFERENCES products(id)

product_tags_demo_dev=# \d tags
Table "public.tags"
┌─────────────┬────────────────────────────────┬───────────┬──────────┬─────────────────────────────┐
│ Column │ Type │ Collation │ Nullable │ Default │
├─────────────┼────────────────────────────────┼───────────┼──────────┼─────────────────────────────┤
│ id │ bigint │ │ not null │ nextval('tags_id_seq'::regc…│
│ │ │ │ │…lass) │
│ name │ character varying(255) │ │ │ │
│ inserted_at │ timestamp(0) without time zone │ │ not null │ │
│ updated_at │ timestamp(0) without time zone │ │ not null │ │
└─────────────┴────────────────────────────────┴───────────┴──────────┴─────────────────────────────┘
Indexes:
"tags_pkey" PRIMARY KEY, btree (id)
"tags_name_index" UNIQUE, btree (name)
Referenced by:
TABLE "products_tags" CONSTRAINT "products_tags_tag_id_fkey" FOREIGN KEY (tag_id) REFERENCES tags(id)

product_tags_demo_dev=# \d products_tags
Table "public.products_tags"
┌────────────┬────────┬───────────┬──────────┬─────────┐
│ Column │ Type │ Collation │ Nullable │ Default │
├────────────┼────────┼───────────┼──────────┼─────────┤
│ product_id │ bigint │ │ not null │ │
│ tag_id │ bigint │ │ not null │ │
└────────────┴────────┴───────────┴──────────┴─────────┘
Indexes:
"products_tags_pkey" PRIMARY KEY, btree (product_id, tag_id)
"products_tags_product_id_index" btree (product_id)
"products_tags_tag_id_index" btree (tag_id)
Foreign-key constraints:
"products_tags_product_id_fkey" FOREIGN KEY (product_id) REFERENCES products(id)
"products_tags_tag_id_fkey" FOREIGN KEY (tag_id) REFERENCES tags(id)

Getting tags to work!

Now comes the fun part, modifying our controllers and contexts to get our tags working!

The first thing we need to do is add a many_to_many relationship on the Product schema like so:

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schema "products" do
field :description, :string
field :name, :string
many_to_many :tags, ProductTagsDemo.Core.Tag, join_through: "products_tags"

timestamps()
end

(Note, that we don’t need to add this relationship on the other side, i.e., Tag to get this working)

Now, we need to modify our Product form to show an input mechanism for tags, the easy way to do this is to ask the users to provide a comma-separated list of tags in an input textbox. A nicer way is to use a javascript library like select2. For us, a text box with comma-separated tags will suffice.

The easiest way to do this is to add a text field like so:

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<%= label f, :tags %>
<%= text_input f, :tags %>
<%= error_tag f, :tags %>

However, as soon as you wire this up you’ll get an error on the /products/new page like below:

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protocol Phoenix.HTML.Safe not implemented for #Ecto.Association.NotLoaded<association :tags is not loaded> of type Ecto.Association.NotLoaded (a struct).

This is telling us that the to_string function can’t convert an Ecto.Association.NotLoaded struct into a string, When you have a relation like a belongs_to or has_one or many_to_many that isn’t loaded on a struct, it has this default value. This is coming from our controller, we can remedy this by changing our action to the following:

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def new(conn, _params) do
changeset = Core.change_product(%Product{tags: []})
render(conn, "new.html", changeset: changeset)
end

Notice the tags: [], we are creating a new product with an empty tags collection so that it renders properly in the form.

Now that we have fixed our form, we can try submitting some tags through this form, However, when you enter any tags and hit Save it doesn’t do anything which is not surprising because we haven’t set up the handling of these tags on the backend yet.

We know that the tags field has comma-separated tags, so we need to do the following to be able to save a product.

  1. Split tags on a comma.
  2. Strip them of whitespace.
  3. Lowercase them to get them to be homogeneous (If you want your tag names to be persisted using the input casing and still treat the uppercased version the same as the lowercased or capitalized versions, you can use :citext (short for case insensitive text) read more about how to set up :citext columns in my blog post about storing username/email in a case insensitive fashion).
  4. Once we have all the tag names we can insert any new tags and then fetch the existing tags, combine them, and use put_assoc to put them on the product.

Step #4 creates a race condition in your code which can happen when 2 requests try to create tags with the same name at the same time. An easy way to work around this is to treat all the tags as new and do an upsert using Repo.insert_all with an on_conflict: :nothing option which adds the fragment ON CONFLICT DO NOTHING to your SQL making your query run successfully even if there are tags with the same name in the database, it just doesn’t insert new tags. Also, note that this function inserts all the tags in a single query doing a bulk insert of all the input tags. Once you upsert all the tags, you can then find them and use a put_assoc to create an association.

This is what ended up as the final Core.create_product function:

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def create_product(attrs \\ %{}) do
%Product{}
|> Product.changeset(attrs)
# use put_assoc to associate the input tags to the product
|> Ecto.Changeset.put_assoc(:tags, product_tags(attrs))
|> Repo.insert()
end

defp parse_tags(nil), do: []

defp parse_tags(tags) do
# Repo.insert_all requires the inserted_at and updated_at to be filled out
# and they should have time truncated to the second that is why we need this
now = NaiveDateTime.utc_now() |> NaiveDateTime.truncate(:second)

for tag <- String.split(tags, ","),
tag = tag |> String.trim() |> String.downcase(),
tag != "",
do: %{name: tag, inserted_at: now, updated_at: now}
end

defp product_tags(attrs) do
tags = parse_tags(attrs["tags"]) # => [%{name: "phone", inserted_at: ..}, ...]

# do an upsert ensuring that all the input tags are present
Repo.insert_all(Tag, tags, on_conflict: :nothing)

tag_names = for t <- tags, do: t.name
# find all the input tags
Repo.all(from t in Tag, where: t.name in ^tag_names)
end

It does the following:

  1. Normalize our tags
  2. Ensure that all the tags are in our database using Repo.insert_all with on_conflict: :nothing in a single SQL query.
  3. Load all the tag structs using the names.
  4. Use put_assoc to associate the tags with the newly created product.
  5. From here Ecto takes over and makes sure that our product has the right association records in the products_tags table

Notice, how through all of our code we haven’t used the products_tags table except for defining the many_to_many relationship in the Product schema.

This is all you need to insert a product with multiple tags, However, we still want to show the tags of a product on the product details page. We can do this by tweaking our action and the Core module like so:

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defmodule Core do
def get_product_with_tags!(id), do: Product |> preload(:tags) |> Repo.get!(id)
end
defmodule ProductTagsDemoWeb.ProductController do
def show(conn, %{"id" => id}) do
product = Core.get_product_with_tags!(id)
render(conn, "show.html", product: product)
end
end

Here we are preloading the tags with the product and we can use it in the view like below to show all the tags for a product:

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Tags: <%= (for tag <- @product.tags, do: tag.name) |> Enum.join(", ") %>

This takes care of creating and showing a product with tags, However, if we try to edit a product we are greeted with the following error:

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protocol Phoenix.HTML.Safe not implemented for #Ecto.Association.NotLoaded<association :tags is not loaded> of type Ecto.Association.NotLoaded (a struct).

Hmmm, we have seen this before when we rendered a new Product without tags, However, in this case, our product does have tags but they haven’t been loaded/preloaded. We can remedy that easily by tweaking our edit action to the following:

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def edit(conn, %{"id" => id}) do
product = Core.get_product_with_tags!(id)
changeset = Core.change_product(product)
render(conn, "edit.html", product: product, changeset: changeset)
end

This gives us a new error:

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lists in Phoenix.HTML and templates may only contain integers representing bytes, binaries or other lists, got invalid entry: %ProductTagsDemo.Core.Tag{__meta__: #Ecto.Schema.Metadata<:loaded, "tags">, id: 1, inserted_at: ~N[2020-05-04 05:20:45], name: "phone", updated_at: ~N[2020-05-04 05:20:45]}

This is because we are using a text_input for a collection of tags and when phoenix tries to convert the list of tags into a string it fails. This is a good place to add a custom input function:

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defmodule ProductTagsDemoWeb.ProductView do
use ProductTagsDemoWeb, :view

def tag_input(form, field, opts \\ []) do
# get the input tags collection
tags = Phoenix.HTML.Form.input_value(form, field)
# render text using the text_input after converting tags to text
Phoenix.HTML.Form.text_input(form, field, value: tags_to_text(tags), opts)
end

defp tags_to_text(tags) do
tags
|> Enum.map(fn t -> t.name end)
|> Enum.join(", ")
end
end

With this helper we can tweak our form to:

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<%= label f, :tags %>
<%= tag_input f, :tags %>
<%= error_tag f, :tags %>
<small class="help-text">tags separated by commas</small>

Note that the text_input has been changed to tag_input.

Now, when we go to edit a product, it should render the form with the tags separated by commas. However, updating the product by changing tags still doesn’t work because we haven’t updated our backend code to handle this. To complete this, we need to tweak the controller and the Core context like so:

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defmodule ProductTagsDemoWeb.ProductController do
def update(conn, %{"id" => id, "product" => product_params}) do
product = Core.get_product_with_tags!(id)
# ... rest is the same
end
end
defmodule ProductTagsDemo.Core do
def update_product(%Product{} = product, attrs) do
product
|> Product.changeset(attrs)
|> Ecto.Changeset.put_assoc(:tags, product_tags(attrs))
|> Repo.update()
end
end

Note that in the controller we are using get_product_with_tags! and in the context, we inserted a line to put_assoc similar to the create_product function which does the same things as create_product.

Astute readers will observe that our create and update product implementation doesn’t rollback newly created tags, when create_product or update_product fails. Let us handle this case and wrap our post!

Ecto provides Ecto.Multi to allow easy database transaction handling. This just needs changes to our context and our view like so:

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defmodule ProductTagsDemo.Core do
alias Ecto.Multi

def create_product(attrs \\ %{}) do
multi_result =
Multi.new()
# use multi to insert all the tags, so the tags are rolled back when there
# is an error in product creation
|> ensure_tags(attrs)
|> Multi.insert(:product, fn %{tags: tags} ->
# This chunk of code remains the same, the only difference is we let
# Ecto.Multi handle insertion of the product
%Product{}
|> Product.changeset(attrs)
|> Ecto.Changeset.put_assoc(:tags, tags)
end)
# Finally, we run all of this in a single transaction
|> Repo.transaction()

# a multi result can be an :ok tagged tuple with the data from all steps
# or an error tagged tuple with the failure step's atom and relevant data
# in this case we only expect failures in Product insertion
case multi_result do
{:ok, %{product: product}} -> {:ok, product}
{:error, :product, changeset, _} -> {:error, changeset}
end
end

# This is identical to `create_product`
def update_product(%Product{} = product, attrs) do
multi_result =
Multi.new()
|> ensure_tags(attrs)
|> Multi.update(:product, fn %{tags: tags} ->
product
|> Product.changeset(attrs)
|> Ecto.Changeset.put_assoc(:tags, tags)
end)
|> Repo.transaction()

case multi_result do
{:ok, %{product: product}} -> {:ok, product}
{:error, :product, changeset, _} -> {:error, changeset}
end
end

# parse_tags is unchanged

# We have created an ensure tags to use the multi struct passed along and the
# repo associated with it to allow rolling back tag inserts
defp ensure_tags(multi, attrs) do
tags = parse_tags(attrs["tags"])

multi
|> Multi.insert_all(:insert_tags, Tag, tags, on_conflict: :nothing)
|> Multi.run(:tags, fn repo, _changes ->
tag_names = for t <- tags, do: t.name
{:ok, repo.all(from t in Tag, where: t.name in ^tag_names)}
end)
end
end

defmodule ProductTagsDemoWeb.ProductView do
use ProductTagsDemoWeb, :view
import Phoenix.HTML.Form

def tag_input(form, field, opts \\ []) do
text_input(form, field, value: tag_value(form.source, form, field))
end

# if there is an error, pass the input params along
defp tag_value(%Ecto.Changeset{valid?: false}, form, field) do
form.params[to_string(field)]
end

defp tag_value(_source, form, field) do
form
|> input_value(field)
|> tags_to_text
end

defp tags_to_text(tags) do
tags
|> Enum.map(fn t -> t.name end)
|> Enum.join(", ")
end
end

Whew, that was long, but hopefully, this gives you a comprehensive understanding of how to handle many_to_many relationships in Ecto and Phoenix.

The source code associated with this blog post can be found at https://github.com/minhajuddin/product_tags_demo

P.S. There is a lot of duplication in our final create_product and update_product functions, try removing the duplication in an elegant way! I’ll share my take on it in the next post!


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