Kong DAO select_by missing?

I’m trying to write some Lua code to find the keyauth_credentials key for a given Consumer. Looking at the source file kong/db/dao/init.lua, it appears that each field in a Kong table should be directly searchable with a :select_by_ method. For the keyauth_credentials table, it has an indexed field consumer_id:

  postgres = {
    up = [[
      CREATE TABLE IF NOT EXISTS "keyauth_credentials" (
        "id"           UUID                         PRIMARY KEY,
        "created_at"   TIMESTAMP WITH TIME ZONE     DEFAULT (CURRENT_TIMESTAMP(0) AT TIME ZONE 'UTC'),
        "consumer_id"  UUID                         REFERENCES "consumers" ("id") ON DELETE CASCADE,
        "key"          TEXT                         UNIQUE
      );

      DO $$
      BEGIN
        CREATE INDEX IF NOT EXISTS "keyauth_credentials_consumer_id_idx" ON "keyauth_credentials" ("consumer_id");
      EXCEPTION WHEN UNDEFINED_COLUMN THEN
        -- Do nothing, accept existing state
      END$$;
    ]],
  },

So I assumed there would be a corresponding select_by_consumer_id method as there’s a select_by_key() one. However, it appears that isn’t the case:

 attempt to call method 'select_by_consumer_id' (a nil value),

How does one tell whether a given field in a Kong table has a corresponding select_by_* method?