Parse the Metadata from an SAP OData V2 Service

This is a work in progress!

Parse the metadata XML describing an SAP OData V2 service and generate Rust modules: one for the Service Document and one for the metadata document.

• <ComplexType> and <EntityType> elements are mapped to Rust structs
• <FunctionImport> functionality will be supported in time, but is not currently available
• Transform Edm.DateTime into chrono::NaiveDateTime using a custom deserializer
• Edm.Decimal fields are handled using the Decimal deserializer in crate rust_decimal; however, this offers only partial support
• OData metadata parsing functionality is only available when the parser crate feature is used
• Generate a metadata module
• Populate the metadata module - I'm working on it...

Usage

You want to write a Rust application to consume the data exposed through an SAP OData V2 service.

The functionality in this crate is invoked by the build script in your application that generates a module of the same name as the OData service you wish to consume.

Together with the code in crate parse-sap-atom-feed the coding in the generated module can then consume entity set data from the OData service.

Declare Build Dependency

In the Cargo.toml of your application, define an entry in [build-dependencies] that points to the parse-sap-odata crate:

[build-dependencies]
parse-sap-odata = { version = "1.2", features = ["parser"]}

Your app will require at least these dependencies:

[dependencies]
chrono = { version = "0.4", features = ["serde"] }
parse-sap-atom-feed = "0.2"
rust_decimal = { version = "1.30", features = ["serde-with-str"]}
serde = { version = "1.0", features = ["derive"] }
uuid = { version = "1.4", features = ["serde"] }

XML Input Files

All metadata XML for the OData services your app consumes must be located in the ./odata directory immediately under your app's top level directory.

Using the demo service GWSAMPLE_BASIC available from SAP's Dev Center server, display the metadata XML for this service, then save that in file ./odata/gwsample_basic.xml.

Create a Build Script

In your app's build script (build.rs), run the generator for your desired OData service:

use parse_sap_odata::parser::gen_src;

fn main() {
    gen_src(
        "gwsample_basic", // metadata_file_name.  The ".xml" extension is assumed
        "GWSAMPLE_BASIC"  // Value of the Namespace attribute on the <Schema> tag
    );
}

More information about Rust build scripts is available on the documentation site.

Generated Output

If cargo detects a build.rs file in your project/crate, then it automatically populates the environment variable OUT_DIR and runs build.rs before compiling your application.

The OUT_DIR variable then points to the directory into which all build script output is written.

The default directory name is target/[debug|release]/build/<your_package_name>/out, and this is where you can find the generated struct declarations for the OData service.

You can specify your own value for OUT_DIR either by passing a value to cargo's --out_dir flag, or by defining your own location in a config.toml file in the ./.cargo directory. See Cargo Configuration for more details.

All generated structs implement at least the following traits #[derive(Clone, Debug, Default)]

Referencing Generated Output

In the source code of your application, use the include!() macro to pull in the generated source code, then bring the generated module into scope with a use command:

// Include the generated code
include!(concat!(env!("OUT_DIR"), "/gwsample_basic.rs"));

use std::str::{self, FromStr};
use gwsample_basic::*;

// Use the BusinessPartner struct for example
fn main() {
    let bp: BusinessPartner = Default::default();
    println!("{:#?}", bp);
}

OData Complex Types

In the event an Entity Type definition uses a complex type, then the complex type is first created as a Rust struct. The field in Rust struct that has this complex type is then defined using this struct.

An example of this is the Address property.

<EntityType Name="BusinessPartner" sap:content-version="1">
  <Key>
    <PropertyRef Name="BusinessPartnerID"/>
  </Key>
  <Property Name="Address" Type="GWSAMPLE_BASIC.CT_Address" Nullable="false"/>

  <!-- SNIP -->

</EntityType>

The Rust struct name is generated by trimming the namespace qualifier and (if present) the CT_ prefix

<ComplexType Name="CT_Address">
  <Property Name="City"        Type="Edm.String" MaxLength="40" sap:label="City" sap:semantics="city"/>
  <Property Name="PostalCode"  Type="Edm.String" MaxLength="10" sap:label="Postal Code" sap:semantics="zip"/>
  <Property Name="Street"      Type="Edm.String" MaxLength="60" sap:label="Street" sap:semantics="street"/>
  <Property Name="Building"    Type="Edm.String" MaxLength="10" sap:label="Building"/>
  <Property Name="Country"     Type="Edm.String" MaxLength="3"  sap:label="Country" sap:semantics="country"/>
  <Property Name="AddressType" Type="Edm.String" MaxLength="2"  sap:label="Address Type"/>
</ComplexType>

So the above XML definition becomes:

#[derive(Clone, Debug, Default)]
pub struct Address {
    pub address_type: Option<String>,
    pub building: Option<String>,
    pub city: Option<String>,
    pub country: Option<String>,
    pub postal_code: Option<String>,
    pub street: Option<String>,
}

OData "Simple" Complex Types

The metadata for the GWSAMPLE_BASIC OData service contains the following complex type:

<ComplexType Name="CT_String">
  <Property Name="String" Type="Edm.String" Nullable="false" sap:creatable="false" sap:updatable="false" sap:sortable="false" sap:filterable="false"/>
</ComplexType>

Allowing for the current situation in which additional attribute values and SAP Annotations are not preserved, this particular type turns out not to be complex at all — its just a String. In such cases, fields declared to be of these "simple" complex types (such as CT_String), are collapsed down to the Rust native type of the single inner property — which in this example is simply a String.

Entity Sets Enum

On the basis that a single OData service exposes a static list of entity sets, and that within the scope of any single request, you will only ever be interacting with a single entity set, it makes sense to treat each entity set name as an enum variant.

Under the <Schema> element in the OData service document, there is an <EntityContainer> element. All entity sets available through this OData service are identified here with their own <EntitySet Name="<some_name>"> tag.

The following naming convention is used: <odata_service_name>Entities.

For example, the entity sets belonging to the OData service GWSAMPLE_BASIC become the following enum:

#[derive(Copy, Clone, Debug)]
pub enum GwsampleBasicEntities {
    BusinessPartnerSet,
    ProductSet,
    SalesOrderSet,
    SalesOrderLineItemSet,
    ContactSet,
    VhSexSet,
    VhCountrySet,
    VhAddressTypeSet,
    VhCategorySet,
    VhCurrencySet,
    VhUnitQuantitySet,
    VhUnitWeightSet,
    VhUnitLengthSet,
    VhProductTypeCodeSet,
    VhBpRoleSet,
    VhLanguageSet,
}

Three convenience functions are then implemented for enum GwsampleBasicEntities:

impl GwsampleBasicEntities {
    pub const fn value(&self) -> &'static str { /* SNIP */ }
    pub fn iterator() -> impl Iterator<Item = GwsampleBasicEntities> { /* SNIP */ }
    pub fn as_list() -> Vec<&'static str> { /* SNIP */ }
}

Entity Set Enum value function

This function returns the name of the entity set variant as a static string slice:

GwsampleBasicEntities::ProductSet::value();   // -> "ProductSet"

Entity Set Enum iterator function

For standard Rust enums such as Option and Result, it makes little sense to attempt to loop over their variants simply because these enums exist specifically to gather together diverse types into a single object. E.G. The Option enum exists to provide a type-safe mechanism for handling the possibility that a variable might not contain a value.

However, an OData service guarantees that the entity set names form an immutable, type-safe list. Therefore, on the basis of this guarantee, the entity set names are placed into an enum that implements an iterator function over its variants.

Entity Set Enum as_list function

By making use of the above iterator and value functions, the as_list function returns the names of the entity sets as a vector of string slices.

Limitations and Issues

1. Currently when generating a Rust struct, only the Name and Type properties are extracted from the XML <EntityType> declaration. Consider how the other XML attribute values and SAP annotations could be made available within the generated Rust struct.

2. The <FunctionImport>, <Association> and <AssociationSet> metadata tags are parsed, but their contents is not currently acted upon.

3. All SAP OData V2 Annotations are processed by serde; however, no action is yet taken based on the annotation values.

4. When calling some of the entity sets in the demo OData service GWSAMPLE_BASIC, certain XML properties are returned whose values are not valid XML. Consequently, when quick_xml attempts to parse such values, it simply throws its toys out the pram and doesn't want to play anymore.

   If you encounter such errors, the raw XML string must first be sanitised before attempting to parse it.

   This functionality is described in the README of parse-sap-odata-demo.

5. The rust_decimal::serde::str deserializer can offer only partial support for fields of Edm.Decimal because it knows nothing about the attributes Precision and Scale:

   <Property Name="Price" Type="Edm.Decimal" Precision="16" Scale="3" sap:unicode="false" sap:unit="CurrencyCode" sap:label="Unit Price"/>
   

   Consequently, these attribute values are not considered when extracting a decimal value.

TODOs

1. Improve support for fields of type Edm.Decimal.
2. Populate the empty OData metadata module.
3. Support Function Imports.