How to query on embed field?

Having such schema:

```
# ProjectUser module

schema "project_users" do
belongs_to :project, Project

embeds_many :stats, ProjectUserStat, on_replace: :delete
end
```

```
# ProjectUserStat module

embedded_schema do
field :date, :string
end
```

Tried
```
ProjectUser
|> where([pu], pu.project_id in ^project_ids)
|> where([pu], pu.stats.date >= ^from)
```
doesnt work:
```
** (Ecto.Query.CompileError) `pu.stats().date()` is not a valid query expression
```

tried with fragments:

```
ProjectUser
|> where([pu], pu.project_id in ^project_ids)
|> where([pu], fragment("stats.date": ["$gte": ^from, "$lte": ^to]))
```

doesnt work:

```
[["stats.date": ["$gte": #BSON.ObjectId<5874d284958c27e7ccd906c3>, "$lte": #BSON.ObjectId<5874d284958c27e7ccd906c4>]]
```

Instead of having strings it converts them to ObjectIds. I'm using ecto 2.0 with this fork https://github.com/michalmuskala/mongodb_ecto/pull/91, so this might be related to that version only, anyway, any ideas?