Thank you @cevado . Exactly what I was looking for.

Anyone having the same issue, what I was looking for was:

from(pa in model,
  select: ^[:id, field],
  where: fragment("?->>'selected_plan' IS NOT NULL", field(pa, ^field))
)

Try moving the interpolation outside of from.

defp with_hierarchical_data(model, field) do
  fd = "#{field}"

  from(pa in model,
    select: [:id, ^field],
    where: fragment("?->>'selected_plan' IS NOT NULL", ^fd)
  )
end

Edit: After thinking about this you may not even need to convert field to a string. Just use the atom and I believe it will convert it for you. The key is that you need to use ? and pass ^field as the 2nd argument to fragment()

defp with_hierarchical_data(model, field) do
  from(pa in model,
    select: [:id, ^field],
    where: fragment("?->>'selected_plan' IS NOT NULL", ^field)
  )
end