title: Optional ecto repos
body: <p>I am having 3 optional repos(not all the databases need to exist). if I am successfully able to connect to any of the repos(I will check all the repos), I need to query(if all the databases are succesfully connected then on all the database) and perform my action.</p>
  
  <p>I have created repos.ex files and corresponding configs.
  after that when I need to query, I would start the repo like this
  <code>{:ok, pid} = DataLayer.DevRepo.start_link(name: :atom)</code>
  Note: (I have tried name: nil as well)</p>
  
  <p>and when I try to query something, following error</p>
  
  <blockquote>
    <p>DataLayer.DevRepo.all DataLayer.User            </p>
  </blockquote>
  
  <pre><code>** (ArgumentError) argument error
  (stdlib) :ets.lookup_element(Ecto.Registry, nil, 3)
  (ecto) lib/ecto/registry.ex:18: Ecto.Registry.lookup/1
  (ecto) lib/ecto/adapters/sql.ex:251: Ecto.Adapters.SQL.sql_call/6
  (ecto) lib/ecto/adapters/sql.ex:426: Ecto.Adapters.SQL.execute_and_cache/7
  (ecto) lib/ecto/repo/queryable.ex:133: Ecto.Repo.Queryable.execute/5
  (ecto) lib/ecto/repo/queryable.ex:37: Ecto.Repo.Queryable.all/4
  </code></pre>
  
  <p>Any help will be really appreciated. Thanks</p>
  
  <p>ecto version <code>2.2.11</code></p>