Parsing nested JSON in Elm

I have this situation

-- this is in post.elm
type alias Model =
  { img : String
  , text : String
  , source : String
  , date : String
  , comments : Comments.Model
  }

-- this is in comments.elm
type alias Model =
  List Comment.Model

-- this is in comment.elm
type alias Model =
  { text : String
  , date : String
  }

I am trying to parse a JSON so formed

{
  "posts": [{
    "img": "img 1",
    "text": "text 1",
    "source": "source 1",
    "date": "date 1",
    "comments": [{
      "text": "comment text 1 1",
      "date": "comment date 1 1"
    }]
  }
}

This is my Decoder

decoder : Decoder Post.Model
decoder =
  Decode.object5
    Post.Model
    ("img" := Decode.string)
    ("text" := Decode.string)
    ("source" := Decode.string)
    ("date" := Decode.string)
    ("comments" := Decode.list Comments.Model)

decoderColl : Decoder Model
decoderColl =
  Decode.object1
    identity
    ("posts" := Decode.list decoder)

It does not work, I am getting

Comments does not expose Model.

How do you expose a type alias?

How do I set up Decoder for my example?