I am using Ecto embeds to store multiple Ecto structs in a Postgres jsonb column.
My issue is that if an embeds_many model in an embedded_schema does not contain
any data, a json value of "model": [] is still inserted into the jsonb column.
For example, I have a Message model with an entities column, and this is
the data that is saved in the database as the entities:
{"id": "18ebed18-568f-4583-b154-4dc44497998b", "urls": [{"id": "c82369d3-de6b-4d39-83ef-b42cf3484603", "url": "example.com", "description": null}], "topics": []}
Whereas, I would like it to be saved as follows, (notice "topics": [] is not present)
{"id": "18ebed18-568f-4583-b154-4dc44497998b", "urls": [{"id": "c82369d3-de6b-4d39-83ef-b42cf3484603", "url": "example.com", "description": null}]}
Here is the Message model
defmodule App.Message do
use App.Web, :model
schema "messages" do
field :text, :string
embeds_one :entities, App.Entities
timestamps()
end
def changeset(struct, params \\ %{}) do
struct
|> cast(params, [:text])
|> cast_embed(:entities)
|> validate_required([:text])
end
end
The Entities model
defmodule App.Entities do
use App.Web, :model
embedded_schema do
embeds_many :urls, App.UrlEntity
embeds_many :topics, App.TopicEntity
end
def changeset(struct, params \\ %{}) do
struct
|> cast(params, [])
|> cast_embed(:urls)
|> cast_embed(:topics)
end
end
The UrlEntity model
defmodule App.UrlEntity do
use App.Web, :model
embedded_schema do
field :url, :string
field :description, :string
end
def changeset(struct, params \\ %{}) do
struct
|> cast(params, [:url, :description])
|> validate_required([:url])
end
end
And the TopicEntity model
defmodule App.TopicEntity do
use App.Web, :model
embedded_schema do
field :name, :string
end
def changeset(struct, params \\ %{}) do
struct
|> cast(params, [:name])
|> validate_required([:name])
end
end
Running the following function will create the message with the entities saved as shown above.
%Message{}
|> Message.changeset(%{
text: "example.com",
entities: %{urls: [%{url: "example.com"}]}
})
|> Repo.insert!
Removing |> cast_embed(:topics) from the Entity changeset results in the same outcome.
There are two reasons it would be desirable for the saved value to not contain "topics": []:
-
Save database disk space
-
Postgres JSONB queries using NOT NULL
Ideally we could query SELECT * FROM messages WHERE entities->'topics' IS NOT NULL;.
But this query will return results if "topics": [] is present.
I would appreciate if anyone had thoughts on how to accomplish this using Ecto embeds, or perhaps a custom Poison encoder.
