A relationship in Power BI is often for when we want a table to filter another table (based on the selection of items in the visuals). This filtering happens in the direction of the relationship. Meaning that if DimCustomer has a one-to-many relationship with the FactSales, and the relationship direction is single directional from the DimCustomer to FactSales, then DimCustomer can filter the FactSales, but not the other way around.
What we see a lot in the Power BI models is that the developer changes the relationship to both-directional so that each of the tables can filter the other table. This looks like a nice feature, but it will come at a big cost.
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