RDF on Elixir

API

As we've said, this is an early version, which can map RDF graphs only. But even the mapping to RDF graphs is limited. You can only map to and from RDF.ex graphs directly, not to graphs in triple stores via SPARQL. Also there are no querying capabilities. So, you'll have to provide the RDF.ex graphs and the identifier by yourself and you'll get back an RDF.ex graph. You can use the serializing capabilities of RDF.ex or the SPARQL client to read and write the RDF.ex graph.

The API for working with the Grax.Schema structs and the individuals of these structs is available on the top-level Grax module and can be applied polymorphically on Grax.Schema structs (with two exceptions).

In the following we will use the example from the last chapter to show the API in action. Here it is again. The example data:

graph =
  """
  @prefix : <http://example.com/> .
  @prefix schema: <https://schema.org/> .
  @prefix foaf: <http://xmlns.com/foaf/0.1/> .

  :User1 
      a schema:Person, :PremiumUser ;
      schema:name "Jane" ;
      schema:email "jane@example.com", "jane@work.com" ;
      foaf:age 30 .

  :Post1
      schema:author :User1 ;
      schema:name "Lorem" ;
      schema:articleBody "Lorem ipsum dolor sit amet, consectetur adipisicing elit. Provident, nihil, dignissimos. Nesciunt aut totam eius. Magnam quaerat modi vel sed, ipsam atque rem, eos vero ducimus beatae harum explicabo labore!" .
  """
  |> RDF.Turtle.read_string!()

The example Grax.Schema structs we've developed in the last chapter:

defmodule User do
  use Grax.Schema

  alias NS.{SchemaOrg, FOAF, EX}

  schema SchemaOrg.Person do
    property name: SchemaOrg.name, type: :string, required: true
    property emails: SchemaOrg.email, type: list_of(:string), required: true
    property age: FOAF.age, type: :integer

    property customer_type: RDF.type, 
               from_rdf: :customer_type_from_rdf,
               to_rdf: :customer_type_to_rdf

    field :password

    link friends: FOAF.friend, type: list_of(User)
    link posts: -SchemaOrg.author, type: list_of(Post)
  end

  def customer_type_from_rdf(types, _description, _graph) do
    {:ok, if(RDF.iri(EX.PremiumUser) in types, do: :premium_user)}
  end

  def customer_type_to_rdf(:premium_user, _user), do: {:ok, EX.PremiumUser}
  def customer_type_to_rdf(_, _), do: {:ok, nil}
end

defmodule Post do
  use Grax.Schema

  alias NS.SchemaOrg

  schema do
    property title: SchemaOrg.name(), type: :string
    property content: SchemaOrg.articleBody(), type: :string
    link author: SchemaOrg.author(), type: User
  end
end

Loading from RDF graphs

You can load a Grax.Schema struct from the description in an RDF graph with the Grax.load/3 function, which expects

  1. the Grax.Schema struct module,
  2. the identifier of the resource to be loaded and
  3. a RDF.Graph with a description of this resource.
iex> Grax.load(User, EX.User1, graph)
{:ok,
 %User{
   __id__: ~I<http://example.com/User1>,
   age: nil,
   customer_type: :premium_user,
   email: ["jane@example.com", "jane@work.com"],
   friends: [],
   name: "Jane",
   password: nil,
   posts: [
     %Post{
       __id__: ~I<http://example.com/Post1>,
       author: #Grax.Link.NotLoaded<link :author is not loaded>,
       content: "Lorem ipsum dolor sit amet, consectetur adipisicing elit. Provident, nihil, dignissimos. Nesciunt aut totam eius. Magnam quaerat modi vel sed, ipsam atque rem, eos vero ducimus beatae harum explicabo labore!",
       title: "Lorem"
     }
   ]
 }}

Unlike most of the other functions working on existing Grax.Schema structs, you have to provide the Grax.Schema module explicitly. For this reason, the Grax.Schema modules provide a dedicated load/2 function on which the order of the resource identifier and the graph arguments is swapped for better pipeline support. So, the following function call is equivalent to the previous call:

graph |> User.load(EX.User1)

There are also bang variants of the both the general Grax.load/3 and the dedicated load/2 functions on the struct modules available, which return the result directly and fail in error cases as usual. But there's another difference between these load variants. The non-bang variant by default performs validations of the data against the schema (described further below), while the non-bang variant does not perform this validation step by default. However, you can control this validation step with the optional :validate option flag independently. Except for the different return type, the load variants just differ in the default value of this :validate option.

Loading values into the structs and performing the validation later is crucial when you want to confront the user with invalid data, eg. in a HTML form for manual cleaning of the data.

The links of a schema will be preloaded as configured in the schema specification. As described in the last chapter, currently only the depth-preloading strategy is implemented. And unless you've configured other preloading depths on the links or the schema, the default preloading depth of one is used, which means you'll get all data and link properties of the loaded resource, but from the linked resources just the data properties. The links of the linked resource won't be loaded.

You can overwrite the preloading options from the schema on a load call with the :depth option.

iex> User.load(graph, EX.User1, depth: 2)
{:ok,
 %User{
   __id__: ~I<http://example.com/User1>,
   age: nil,
   customer_type: :premium_user,
   email: ["jane@example.com", "jane@work.com"],
   friends: [],
   name: "Jane",
   password: nil,
   posts: [
     %Post{
       __id__: ~I<http://example.com/Post1>,
       author: %User{
         __id__: ~I<http://example.com/User1>,
         age: nil,
         customer_type: :premium_user,
         email: ["jane@example.com", "jane@work.com"],
         friends: [],
         name: "Jane",
         password: nil,
         posts: #Grax.Link.NotLoaded<link :posts is not loaded>
       },
       content: "Lorem ipsum dolor sit amet, consectetur adipisicing elit. Provident, nihil, dignissimos. Nesciunt aut totam eius. Magnam quaerat modi vel sed, ipsam atque rem, eos vero ducimus beatae harum explicabo labore!",
       title: "Lorem"
     }
   ]
 }}

Explicit preloading

There still can be occasions when a manual preloading is needed:

  • when the defaults, either from the schema or the :depth keyword on load/3 weren't sufficient
  • when a circle occurred during preloading, aborting the preloading further down
  • or you simple discover later on, that you need further data

You can do a manual preload with the Grax.preload/3 function.

iex> user = User.load!(graph, EX.User1)
%User{
  __id__: ~I<http://example.com/User1>,
  age: nil,
  customer_type: :premium_user,
  email: ["jane@example.com", "jane@work.com"],
  friends: [],
  name: "Jane",
  password: nil,
  posts: [
    %Post{
      __id__: ~I<http://example.com/Post1>,
      author: #Grax.Link.NotLoaded<link :author is not loaded>,
      content: "Lorem ipsum dolor sit amet, consectetur adipisicing elit. Provident, nihil, dignissimos. Nesciunt aut totam eius. Magnam quaerat modi vel sed, ipsam atque rem, eos vero ducimus beatae harum explicabo labore!",
      title: "Lorem"
    }
  ]
}

iex(4)> Grax.preload(user, graph, depth: 2)
{:ok,
 %User{
   __id__: ~I<http://example.com/User1>,
   age: nil,
   customer_type: :premium_user,
   email: ["jane@example.com", "jane@work.com"],
   friends: [],
   name: "Jane",
   password: nil,
   posts: [
     %Post{
       __id__: ~I<http://example.com/Post1>,
       author: %Example.User{
         __id__: ~I<http://example.com/User1>,
         age: nil,
         comments: #Grax.Link.NotLoaded<link :comments is not loaded>,
         customer_type: :premium_user,
         email: ["jane@example.com", "jane@work.com"],
         name: "Jane",
         password: nil,
         posts: #Grax.Link.NotLoaded<link :posts is not loaded>
       },
       content: "Lorem ipsum dolor sit amet, consectetur adipisicing elit. Provident, nihil, dignissimos. Nesciunt aut totam eius. Magnam quaerat modi vel sed, ipsam atque rem, eos vero ducimus beatae harum explicabo labore!",
       title: "Lorem"
     }
   ]
 }}

Note, that this function essentially overwrites everything accept the data properties on the root node.

Creation from scratch

When an RDF description of a resource does not exist yet, but should be created in the application, the build functions can be used. Similar to the load functions there is also a bang-variant and as it requires the Grax.Schema module as the first argument too, there are are also dedicated functions of both build functions without this argument on the Grax.Schema modules available. The other required argument then remains the resource identifier.

iex> User.build!(EX.User2)
%User{
  __id__: ~I<http://example.com/ex>,
  age: nil,
  customer_type: nil,
  email: [],
  friends: [],
  name: nil,
  password: nil,
  posts: []
}

You can also pass initial values as a map or keyword list to each of the build functions:

iex> user = User.build!(EX.User2, 
...>   name: "John", 
...>   email: "john@example.com", 
...>   password: "secret")
 %User{
  __id__: ~I<http://example.com/User2>,
  age: nil,
  customer_type: nil,
  email: ["john@example.com"],
  friends: #Grax.Link.NotLoaded<link :friends is not loaded>,
  name: "John",
  password: "secret",
  posts: #Grax.Link.NotLoaded<link :posts is not loaded>
}

Again, the non-bang variant performs validations, while the bang variant doesn't.

Schema-conformant updates

You can set specific property values of Grax.Schema structs with the Grax.put/3 function.

iex> Grax.put(user, :age, 42)
{:ok,
 %User{
   __id__: ~I<http://example.com/User2>,
   age: 42,
   customer_type: nil,
   email: ["john@example.com"],
   friends: [],
   name: "John",
   password: "secret",
   posts: []
 }}

All preexisting values of a property will be overwritten.

If the value doesn't match the type of the property specified in the schema an :error with a detailed error struct is returned.

iex> Grax.put(user, :age, "old")
{:error,
 %Grax.Schema.TypeError{
   message: "value \"old\" does not match type RDF.XSD.Integer",
   type: RDF.XSD.Integer,
   value: "old"
 }}

The non-bang variant doesn't perform validations. But even without that, you should always prefer the Grax.put!/3 function over Elixir's Map.put/3 function or other methods for updating structs, since it will behave schema-aware. For example, if you put a single value on a property with multiple possible values it will be put into a list:

iex> Grax.put!(user, :email, "john@doe.com")
%User{
  __id__: ~I<http://example.com/User2>,
  age: 42,
  customer_type: nil,
  email: ["john@doe.com"],
  friends: [],
  name: "John",
  password: "secret",
  posts: []
}

Nested structs can be set by providing a Grax.Schema struct accordingly.

iex> Grax.put!(user, :posts, Post.build!(EX.Post2, title: "Foo"))
%User{
  __id__: ~I<http://example.com/User2>,
  age: 42,
  customer_type: nil,
  email: ["john@example.com"],
  friends: [],
  name: "John",
  password: "secret",
  posts: [
    %Post{
      __id__: ~I<http://example.com/Post2>,
      author: nil,
      content: nil,
      title: "Foo"
    }
  ]
}

The Grax.put/2 and Grax.put!/2 functions also allow to set multiple values at once with a map or keyword list.

iex> Grax.build!(user, 
...>   email: ["john@doe.com" | user.email],
...>   age: user.age + 1)
%User{
  __id__: ~I<http://example.com/User2>,
  age: 43,
  customer_type: nil,
  email: ["john@doe.com", "john@example.com"],
  friends: [],
  name: "John",
  password: "secret",
  posts: []
}

Validation against the schema

A Grax.Schema struct can be validated with the Grax.validate/1 function which either returns the given Grax.Schema struct unchanged in an :ok tuple. Otherwise an :error tuple with a Grax.ValidationError containing a collection of all failed validations.

iex> User.build!(EX.User2, 
...>   name: ["John", "JD"],
...>   age: "old")
...> |> Grax.validate()
{:error,
 %Grax.ValidationError{
   errors: [
     name: %Grax.Schema.TypeError{
       message: "value [\"John\", \"JD\"] does not match type RDF.XSD.String",
       type: RDF.XSD.String,
       value: ["John", "JD"]
     },
     email: %Grax.Schema.CardinalityError{
       message: "[] does not match cardinality {:min, 1}",
       cardinality: {:min, 1}, 
       value: []
     },
     age: %Grax.Schema.TypeError{
       message: "value \"old\" does not match type RDF.XSD.Integer",
       type: RDF.XSD.Integer,
       value: "old"
     }
   ]
 }}

Mapping to RDF graphs

With the Grax.to_rdf/1 function finally, you can map a Grax.Schema struct to a RDF.Graph.

iex> Grax.to_rdf(user)
{:ok,
 #RDF.Graph<name: nil
  @prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
  @prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
  @prefix xsd: <http://www.w3.org/2001/XMLSchema#> .
  
  <http://example.com/Post2>
      <https://schema.org/author> <http://example.com/User2> ;
      <https://schema.org/name> "Foo" .
  
  <http://example.com/User2>
      a <https://schema.org/Person> ; 
      <https://schema.org/email> "john@doe.com", "john@example.com" ;
      <https://schema.org/name> "John" ;
      <http://xmlns.com/foaf/0.1/age> 43 .
>}

The options given to Grax.to_rdf/2 as the optional second argument are passed-through as options to the RDF.Graph.new/2 call used for the creation of the graph. This allows you to set the name of the graph, define some prefixes, a base URI etc.

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