Ecto will not insert a :date field into the database - why? - postgresql

Question

I am facing an issue while learning Elixir & Ecto. The idea is to build a standard posts/comments page to understand how the basics work. I am at a point where I have schemas defined, a migration written and encounter an error when trying to insert data into the database (PostgreSQL) via the Repo. I have done a fair deal of web searching and documentation reading, which leads me to believe it's a scenario that should just work and I am making a stupid mistake somewhere, which I just can't see.
They are defined as follows:
lib/hello/schemas.ex
defmodule Hello.PostAuthor do
use Ecto.Schema
schema "post_authors" do
field :name, :string
end
end
defmodule Hello.CommentAuthor do
use Ecto.Schema
schema "comment_authors" do
field :name, :string
end
end
defmodule Hello.Comment do
use Ecto.Schema
schema "comments" do
has_one :author, Hello.CommentAuthor
field :date, :date
field :body, :string
end
end
defmodule Hello.Post do
use Ecto.Schema
schema "posts" do
has_one :author, Hello.PostAuthor
field :date, :date
field :body, :string
has_many :comments, Hello.Comment
end
end
as you can see, I have two fields with :date type - on post and comment schemas. The corresponding migration is as follows:
defmodule Hello.Repo.Migrations.CreatePosts do
use Ecto.Migration
def change do
create table(:post_authors) do
add :name, :string
end
create table(:comment_authors) do
add :name, :string
end
create table(:comments) do
add :author, references(:comment_authors)
add :date, :date
add :body, :string
end
create table(:posts) do
add :author, references(:post_authors), null: false
add :date, :date
add :body, :string
add :comments, references(:comments)
timestamps()
end
end
end
Now, when I start iex -S mix I can successfully create all structs:
iex(1)> post_author = %Hello.PostAuthor{name: "John"}
%Hello.PostAuthor{
__meta__: #Ecto.Schema.Metadata<:built, "post_authors">,
id: nil,
name: "John"
}
iex(2)> comment_author = %Hello.PostAuthor{name: "Adam"}
%Hello.PostAuthor{
__meta__: #Ecto.Schema.Metadata<:built, "post_authors">,
id: nil,
name: "Adam"
}
iex(3)> comment = %Hello.Comment{author: comment_author, date: ~D[2019-01-01], body: "this is a comment"}
%Hello.Comment{
__meta__: #Ecto.Schema.Metadata<:built, "comments">,
author: %Hello.PostAuthor{
__meta__: #Ecto.Schema.Metadata<:built, "post_authors">,
id: nil,
name: "Adam"
},
body: "this is a comment",
date: ~D[2019-01-01],
id: nil
}
iex(4)> post = %Hello.Post{author: post_author, date: ~D[2019-01-01], body: "this is a post", comments: [comment]}
%Hello.Post{
__meta__: #Ecto.Schema.Metadata<:built, "posts">,
author: %Hello.PostAuthor{
__meta__: #Ecto.Schema.Metadata<:built, "post_authors">,
id: nil,
name: "John"
},
body: "this is a post",
comments: [%Hello.Comment{
__meta__: #Ecto.Schema.Metadata<:built, "comments">,
author: %Hello.PostAuthor{
__meta__: #Ecto.Schema.Metadata<:built, "post_authors">,
id: nil,
name: "Adam"
},
body: "this is a comment",
date: ~D[2019-01-01],
id: nil
}],
date: ~D[2019-01-01],
id: nil
}
The problem arises when I call Hello.Repo.insert(post) (where post is the struct representing the Hello.Post schema). I receive what looks like serialization error:
iex(8)> Hello.Repo.insert(post) [debug] QUERY OK db=0.1ms
begin []
[debug] QUERY ERROR db=1.6ms
INSERT INTO "posts" ("body","date") VALUES ($1,$2) RETURNING "id" ["this is a post", ~D[2019-01-01]]
[debug] QUERY OK db=0.1ms
rollback []
** (DBConnection.EncodeError) Postgrex expected a binary, got ~D[2019-01-01]. Please make sure the value you are passing matches the definition in your table or in your query or convert the value accordingly.
(postgrex) lib/postgrex/type_module.ex:897: Postgrex.DefaultTypes.encode_params/3
(postgrex) lib/postgrex/query.ex:75: DBConnection.Query.Postgrex.Query.encode/3
(db_connection) lib/db_connection.ex:1148: DBConnection.encode/5
(db_connection) lib/db_connection.ex:1246: DBConnection.run_prepare_execute/5
(db_connection) lib/db_connection.ex:540: DBConnection.parsed_prepare_execute/5
(db_connection) lib/db_connection.ex:533: DBConnection.prepare_execute/4
(postgrex) lib/postgrex.ex:198: Postgrex.query/4
(ecto_sql) lib/ecto/adapters/sql.ex:666: Ecto.Adapters.SQL.struct/10
(ecto) lib/ecto/repo/schema.ex:651: Ecto.Repo.Schema.apply/4
(ecto) lib/ecto/repo/schema.ex:262: anonymous fn/15 in Ecto.Repo.Schema.do_insert/4
(ecto) lib/ecto/repo/schema.ex:916: anonymous fn/3 in Ecto.Repo.Schema.wrap_in_transaction/6
(ecto_sql) lib/ecto/adapters/sql.ex:898: anonymous fn/3 in Ecto.Adapters.SQL.checkout_or_transaction/4
(db_connection) lib/db_connection.ex:1415: DBConnection.run_transaction/4
This is where I am lost. Both the schema and the migration are expecting a :date . I believe that ~D[2019-01-01] is a date. PostgreSQL defines date as a 4 byte binary value. I am expecting Ecto.Adapters.Postgres to translate elixir date struct into the Postgres binary value. This is not happening. Why?

Answer 1

Struct itself is just raw data. You should go through Ecto.Changeset as shown in the documentation, specifically to all types to be cast to the respective DB types with Ecto.Changeset.cast/4.
The conversion will be done automagically, but you need to explicitly call cast/4 (hence the Changeset,) otherwise the adapter has no idea of how to convert your ecto types.