How to get the vice-versa relation to a many-to-one relation?

Problems and solutions
1

I already had this question once, but slack isn’t showing me the result.
I have a many-to-one relation from table a to b
How can I get the vice versa reference to the related record from b to a??

2

one-to-many is the vice versa. So:

article.cfc

property name="category" relationship="many-to-one" relatedto="category";
property name="topic"    relationship="many-to-one" relatedto="topic";
// ...

category.cfc

property name="articles" relationship="one-to-many" relatedto="article" relationshipkey="category";

Example query utilisation

selectData( objectName="category", filter={ "articles.topic"=topic } );
selectData(
      objectName   = "category"
    , selectFields = [ "category.label as cat", "articles.id", "articles.label", "articles$topic.label as topic" ]
    , filter       = { id=categoryId }
);

https://docs.preside.org/devguides/dataobjects.html#one-to-many-relationships

3

one-to-many relationships are also commonly used for formula fields such as number_articles. From Preside 10.23 there are also aggregate helpers that make this easy and more query efficient:

property name="articles" relationship="one-to-many" relatedto="article" relationshipkey="category";
property name="article_count" formula="agg:count{ articles.id }" type="numeric";

https://docs.preside.org/devguides/dataobjects.html#aggregate-functions-in-formula-fields

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