"constraint error when attempting to delete struct:"

* comments_post_id_fkey (foreign_key_constraint)

If you would like to stop this constraint violation from raising an
exception and instead add it as an error to your changeset, please
call foreign_key_constraint/3 on your changeset with the constraint
:name as an option.

def delete(conn, %{"id" => post_id}) do
    Repo.get!(Post, post_id) |> Repo.delete!
    |> put_flash(:info, "Post Deleted")
    |> redirect(to: Routes.post_path(conn, :index))

Where to add the constraint to fix this error?

You’ll need to use a Changeset to delete the record.
The Changeset.change/2 function can be used to wrap an existing struct in a Changeset.

|> Repo.get!(post_id) 
|> Changeset.change()
|> Changeset.foreign_key_constraint(:id, name: "comments_post_id_fkey", message: "can't be deleted, it is still in use")
|> Repo.delete

Here’s the output I get in an iex session:

|> Repo.get(1) 
|> Changeset.change() 
|> Changeset.foreign_key_constraint(:artist_id, name: "FK_AlbumArtistId", message: "can't be deleted, it is still in use") 
|> Repo.delete()

[debug] QUERY OK source="Artist" db=2.3ms idle=55.4ms
SELECT A0."ArtistId", A0."Name" FROM "Artist" AS A0 WHERE (A0."ArtistId" = $1) [1]

[debug] QUERY ERROR db=2.0ms idle=58.0ms
DELETE FROM "Artist" WHERE "ArtistId" = $1 [1]

   action: :delete,
   changes: %{},
   errors: [
     artist_id: {"is being used by another table",
      [constraint: :foreign, constraint_name: "FK_AlbumArtistId"]}
   data: #Artist<>,
   valid?: false