OData Representation

Introduction

This document describes how entities are represented in a published OData service.

Attributes

¹ When using Excel to import an OData source, long numbers may seem cut off. This is due to a restriction in the data type Microsoft uses. For more information, see Last digits are changed to zeroes when you type long numbers in cells of Excel.
² Even though the binary data type is not supported, the FileDocument and Image system entities are supported and represented as Base64-encoded strings with the Edm.Binary type.
³ When the string attribute has a limited length, the MaxLength attribute is specified. ⁴ The enumerations feature was introduced in Studio Pro 9.24.0. In earlier versions, all enumeration attributes were published as strings.

Additionally, the updated field for an entry in OData comes from the system changedDate attribute of an entity. If this attribute is not available (because it is not exposed, the user does not have access rights, or it is empty in database), the default date (1-1-1970) will be used.

Representation of ID

Each entity has an ID, which is not shown as an attribute in the domain model. This is indicated in the service's metadata.

In OData 4, IDs are annotated with vocabulary annotation Com.Mendix.IsAttribute with value false. The term for this vocabulary annotation is included in the metadata.

In OData 3, IDs are marked with isAttribute="false", using a Mendix-specific XML attribute in the http://www.mendix.com/Protocols/MendixData namespace.

Large Long Values and JavaScript

A Mendix Long can hold values up to 2^63 − 1. In JavaScript, the largest safe integer is Number.MAX_SAFE_INTEGER, which is 2^53 − 1 (9007199254740991).

When a client parses a larger integer as a JavaScript number, it can lose precision, so it no longer has the exact Long value. This can affect data in a browser. For example, consider the following OData response:

{"@odata.context":"...snip...","value":[{"LongValue":12345678901234567}]}

Depending on the browser and how it parses and displays the response, the value of LongValue can appear as 12345678901234568.

This loss of precision is not caused by Mendix. However, you should take it into account when modeling published services. To avoid it, consider the following:

• In the domain model, change the attribute type to String.
• Do not expose the Mendix ID. Large values frequently occur in IDs because of the way they are constructed.

Associations

In the settings of the OData service, you can choose how associations are represented. There are two options, which are described below.

As a Link

When you represent associations as links, the data of associated objects can be included in the response by using the $expand query parameter. For more information, see the Retrieving Associated Objects section of Supported OData Operations.

This means that you can only expose an association when the entity on the other side is a resource of this service as well. This also means that you cannot publish the same entity more than once in the same service (because in that case, it would not be clear where the link should point to).

Using this method, you can expose both sides of the association and you can expose many-to-many associations.

As an Associated Object ID

When you choose to represent associations as an associated object ID, the ID of the associated object is represented as an Edm.Int64 property. If the association refers to more than one object, you can not expose it from that side.