In my `Room` model, I have an attribute named `available_days`, which is being stored as an array. 

For example:

```
Room.first.available_days
=> ["wed", "thurs", "fri"]

```

What is the best way to find all Rooms where the size of the array is equal to 3?

I've tried something like

```
Room.where('LENGTH(available_days) = ?', 3)

```

with no success. 

**Update:** the data type for `available_days` is a string, but in order to store an array, I am serializing the attribute from my model:

app/models/room.rb

```
serialize  :available_days

```