Making a belongs_to with substring as foreign key?

Hi,

I have two related entities : Phones and TACs. TACs represent phone models, they are identified by a TAC number which is an 8 digits string. Phones are identified by an IMEI number which is a 15-16 digits string. The first 8 chars of an IMEI number are always a TAC number.

I would like to be able to do something like that so that I can load my phones with their tac data.

schema "tacs" do
  field(:tac, :string) # exactly 8 chars
  field(:model_name, :string)
  [...]
end

schema "mobile_phones" do
  field(:imei, :string) # 15 or 16 chars
  [...]
  belongs_to(:tacs, Tac, foreign_key: [... the first 8 chars of field :imei ...])
end

I would like to avoid making a DB view. How could I tackle that problem in pure Elixir ?