Thing

# field {:content, "text", &cast_string/1}
#       ^          ^                ^
# struct field     |                |
#     path to data in the source    |
#                            casting function

This says in the source data there will be a field called :text. When creating a struct we should get the data under that field and pass it too cast_string/1. The result of that function will be put in the resultant struct under the key :content.

There are 5 kinds of struct fields we could want:

1. field - The value will be a casted value from the source data.
2. list_of - The value will be a list of casted values created from the source data.
3. has_one - The value will be created from a nested data schema (so will be a struct)
4. has_many - The value will be created by casting a list of values into a data schema. (You end up with a list of structs defined by the provided schema). Similar to has_many in ecto
5. aggregate - The value will be a casted value formed from multiple bits of data in the source.

Available options are:

• :optional? - specifies whether or not the field in the struct should be included in the @enforce_keys for the struct. By default all fields are required but you can mark them as optional by setting this to true. This will also be checked when creating a struct with DataSchema.to_struct/2 returning an error if the required field is null.

For example:

defmodule Sandwich do
  require DataSchema

  DataSchema.data_schema(field: {:type, "the_type", &{:ok, String.upcase(&1)}, optional?: true})
end

To see this better let’s look at a very simple example. Assume our input data looks like this:

source_data = %{
  "content" => "This is a blog post",
  "comments" => [%{"text" => "This is a comment"}, %{"text" => "This is another comment"}],
  "draft" => %{"content" => "This is a draft blog post"},
  "date" => "2021-11-11",
  "time" => "14:00:00",
  "metadata" => %{"rating" => 0}
}