Adding calculated field on save

fireproofsocks

I’m working on a table to store shipping addresses. I would like to add a fingerprint of each address so that I can more easily tell whether or not a given address has already been added. For simplicity, let’s assume that I’ve worked out a nice fingerprinting algorithm to normalize my addresses and identify them via a single string… let’s say this function is named simply fingerprint(address).

My question is about Ecto and changesets: how and where can I add this field to the database record when I save it? I’m unclear on how the cast and other functions are working inside the changeset function.

If I try to save a new address and its fingerprint matches an existing address, the save operation should fail.
If I try to edit an existing address and change it so it matches an existing address, the update operation should fail.

Any help on this would be appreciated… I’m having a hard time wrapping my head around where to add this and how to think about it in an idiomatic way. Thanks!

12 comments

#ecto #changesets

2 4109 12

2018-06-19 15:35:57 UTC

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kokolegorille

Probably something like this…

  @doc false
  def changeset(whatever, attrs) do
    whatever
    |> changeset(attrs)
    |> cast(attrs, @fields)
    |> validate ...
    |> fingerprint()
    |> unique_constraint(:fingerprint, message: "Fingerprint already in use")
  end

  defp fingerprint(changeset) do
    case changeset do
      %Ecto.Changeset{valid?: true, changes: %{address: address}} ->
        put_change(changeset, :fingerprint, create_fingerprint(address))
      _ ->
        changeset
    end
  end

  defp create_fingerprint(address) do
    # Implement your fingerprint here
  end

It is not tested, and fields might not correspond to your structure, but You can get the idea… the 2 last lines from changeset are important.

Post #1

kokolegorille

This should not work… as it should be a field

You should use

%Ecto.Changeset{valid?: true, changes: %{street1: street1, street2: street2...}}

# or

%Ecto.Changeset{valid?: true, changes: address}

In the example I gave, whatever is the shemas, and address is just a field of %Whatever{}

You might find ecto changeset structure here. From docs…

changes - The changes from parameters that were approved in casting

Post #9

kokolegorille

You might show your schema, and migration file. Be sure :fingerprint field exists, as string…

You might also insert some IO.inspect in your pipeline to show the changeset, like so

  def changeset(whatever, attrs) do
    whatever
    |> changeset(attrs)
    |> cast(attrs, @fields)
    |> validate ...
    |> IO.inspect()
    |> fingerprint()
    |> IO.inspect()
    |> unique_constraint(:fingerprint, message: "Fingerprint already in use")
  end

The function fingerprint take a changeset, and if it is valid, it should persists the change.

If it is clearer, You could have used pattern matching, like so.

defp fingerprint(%Ecto.Changeset{valid?: true, changes: %{address: address}} = changeset) do
  put_change(changeset, :fingerprint, create_fingerprint(address))
end
defp fingerprint(changeset), do: changeset

BTW Try to change the name of the function to generate_fingerprint(), just to be sure it does not conflict with the field name.

Post #4

Last Post!

fireproofsocks

Thank you all! I got it working and I learned a lot in the process – inspecting the changeset with IO.inspect() was helpful because I could see the diffs and where fields were being included in or omitted from the changeset. I ran into a bit of trouble because I had set a default value for country, so that would cause that key to be missing from the changeset. Similarly, with the optional “street2” value, if it was sent with a blank value or omitted entirely, the changeset would end up without that field.

Post #12