> Isn't that's just having a property/field on a struct/class/type/object that contains the list of tags?

I have never used the library, but it seems you get a lot more with just 2-3 lines of configuration (e.g. for a tag context named „interests“):

- The ability to find the n most or least used tags with a single method call - The ability to track which user tagged a record - The ability to find records with the same tags - Calculation of tag statistics for a tag cloud

Now, all that would certainly be possible with EF. But many libraries for Rails give developers a very simple way to configure a feature, and deliver very expressive methods to use the feature. Which is an important property imo, since it often makes obvious what the code does, even for newcomers.

This is probably an effect from Ruby itself, where the standard library is quite expansive and has many often-used patterns built in. For example, calculating an arrays maximum value is just

    arr = [1,2,3,4]
    arr.max

Meanwhile, in JS:

    arr = [1,2,3,4]
    let max = arr[0];
    for (let i = 1; i < arr.length; i++) {
        if (arr[i] > max) max = arr[i];
    }

And to address the README of the above library: I think it is a bit confusing because it starts with a comparison with another library, expecting readers to already know how tagging worked there.