You could use `pluck_select` instead (it is defined in `GraphStructures::SqlHelper`).
With it included the line would look like that: 

```
@file_extensions_distributed ||= pluck_select(
  'SELECT extension FROM ontology_file_extensions WHERE distributed = \'true\'', :extension)
```
However even better would probably be using arel. But that would probably complicate things in this case.