Add Virtual Fields With Ecto Schemas as an elixir til

elixir/virtual-fields-with-ecto-schemas.md

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+# Virtual Fields With Ecto Schemas
+
+If you'd like to include a particular key-value pair in an Ecto changeset,
+it needs to be included as a field in the schema. In the case of something
+akin to a password field, you want to be able to perform validations against
+it, but the password itself does not have a column in the database. In other
+words, you want to use the password in memory as part of the validation
+process but not save it to the database. To accomplish this, you need to
+specify that it is a `virtual` field.
+
+```elixir
+schema "users" do
+  field :username, :string
+  field :password_digest, :string
+  field :password, :string, virtual: true
+end
+```
+
+With that schema, you can then validate the `:password` and transform it
+into the corresponding `:password_digest` field.
+
+```elixir
+def registration_changeset(model, params) do
+  model
+  |> changeset(params)                  # do other standard validations
+  |> cast(params, [:password])          # include :password in the changeset
+  |> validate_length(:password, min: 8) # validations
+  |> put_pass_hash()                    # transform into :password_digest
+end
+```