1

This is my database.fsb file

table Author {
    /// objectbox:id
    id:ulong;
    text:string;
}

table Book{
    /// objectbox:id
    id: ulong;
    name: string;

    /// objectbox:relation(name=authors,to=Author)
    authors:ulong;
}

What type I must use for authors in Book table to make a many-to-many relation between book and authors?

Mohammad Golkar
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2 Answers2

-1

There is no built-in many-to-many relationship in FlatBuffers, as that would constitute a graph, and FlatBuffers cannot do cycles, only trees/dags.

You could use authors:[Author], but you cannot then also add a books:[Book] to Author as that would create a cycle. So you have to pick one of the two, and re-construct the opposite relation when you're reading the data.

Aardappel
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  • This is about ObjectBox "extensions" to the plain FlatBuffers schema as outlined here: https://cpp.objectbox.io/entity-annotations E.g. ObjectBox has "real" many-to-many relations. – Markus Junginger Jan 11 '22 at 08:27
  • @MarkusJunginger I have no idea what ObjectBox is, I only know FlatBuffers. But regardless of what ObjectBox does, it will not be able to overcome the limitations of the format I mentioned above. – Aardappel Jan 11 '22 at 17:42
  • TLDR: ObjectBox is a database that "happens" to use FlatBuffers (it has quite nice properties for the DB actually). We're using comments for ObjectBox specific annotations in the lack of real annotations in the plain FB schema. – Markus Junginger Jan 12 '22 at 07:53
-1

First some background: a many-to-many relation in the ObjectBox database holds its data "outside" of the entities (in a separate data "bucket"; the pendant to a "table" of a relational database).

Thus, that relation annotation is already sufficient.

You did not indicate which language you are using, but both C and C++ ObjectBox APIs do not have wrapper types for relations (e.g. lists as used in higher level languages). Instead, you must use the low-level functions obx_cursor_rel_put, obx_cursor_rel_remove, and obx_cursor_rel_ids. Or, the box-based functions like obx_box_rel_put.

To give you an impression, check the following gist: https://gist.github.com/greenrobot/552f560a92737ec045b0efdc2122fd62 - you cannot run it as because it lacks the testing context, but should give you an idea of how to use it.

Markus Junginger
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