Dealing with an array in phoenix

First, attrs is not an array, it’s a map. And you can use:

attrs = Map.replace(attrs, :user1_id, 5).

Also, about Map.update!/3, that’s not how the capture operator (&) works. The anonymous function in that function only provides a single parameter (the current value). If you use &(&5), you’re trying to access the fifth parameter being provided, which does not exist in this case. Doing it like this Map.update!(attrs, :user1_id, &(&1 + 1)) would means that i’m taking the current value, and adding one to it.

One way to update a value in a map is to replace it using Map.put/3. It will replace or add any key-value pair. There is also Map.put_new if you don’t want to create a new key-value- pair.

attrs = %{ "user_id" => 1 }

 Map.put(attrs, "user_id", 5)
# %{ "user_id" => 5 }

attrs["user1_id"] will not work with %{user1_id: 1, user2_id: 2} as the first expression uses string keys while the map is defined with atom keys.

While this is very true, I want to point out the likelihood that OP has the opposite issue: attrs likely uses string keys, coming from a Phoenix form, and can’t be updated or accessed using atoms.