Delay encoding nested queries (#171)

When using an inner query or a join as data source in the `from`
macro, don't encode it to a JSON-like map immediately.  Instead, do it
when we encode the outer query.

This should make it easier to modify the inner query as well as the
outer query before encoding.

Author: legoscia

lib/panoramix/query.ex

@@ -142,21 +142,19 @@ defmodule Panoramix.Query do
 
           _ ->
             # Are we creating a new query from scratch, given some kind of datasource?
-            %Panoramix.Query{data_source: Panoramix.Query.datasource(source)}
+            %Panoramix.Query{data_source: source}
         end
 
       Map.merge(query, Map.new(query_fields))
     end
   end
 
-  @doc nil
-  # exported only so that the `from` macro can call it.
-  def datasource(datasource) when is_binary(datasource) do
+  defp datasource(datasource) when is_binary(datasource) do
     # We're using a named datasource as the source for the query
     datasource
   end
 
-  def datasource(%{type: :query, query: nested_query} = datasource) do
+  defp datasource(%{type: :query, query: nested_query} = datasource) do
     # The datasource is a nested query. Let's convert it to JSON if needed
     nested_query_json =
       case nested_query do
@@ -171,7 +169,7 @@ defmodule Panoramix.Query do
     %{datasource | query: nested_query_json}
   end
 
-  def datasource(%{type: :join, left: left, right: right} = datasource) do
+  defp datasource(%{type: :join, left: left, right: right} = datasource) do
     # A join between two datasources.
     # A named datasource and a recursive join can only appear on the
     # left side, but let's let Druid enforce that.
@@ -180,7 +178,7 @@ defmodule Panoramix.Query do
     %{datasource | left: left_datasource, right: right_datasource}
   end
 
-  def datasource(%{type: type} = datasource) when is_atom(type) do
+  defp datasource(%{type: type} = datasource) when is_atom(type) do
     # Some other type of datasource. Let's include it literally.
     datasource
   end
@@ -852,7 +850,7 @@ defmodule Panoramix.Query do
       analysisTypes: query.analysis_types,
       bound: query.bound,
       context: query.context,
-      dataSource: query.data_source,
+      dataSource: datasource(query.data_source),
       dimension: query.dimension,
       dimensions: query.dimensions,
       filter: query.filter,