properly underscore types

cereal_ex.iml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<module type="ELIXIR_MODULE" version="4">
+  <component name="NewModuleRootManager" inherit-compiler-output="true">
+    <exclude-output />
+    <content url="file://$MODULE_DIR$" />
+    <orderEntry type="inheritedJdk" />
+    <orderEntry type="sourceFolder" forTests="false" />
+  </component>
+</module>

lib/cereal/utils.ex

@@ -10,7 +10,7 @@ defmodule Cereal.Utils do
     |> List.last()
     |> String.replace("Serializer", "")
     |> String.replace("View", "")
-    |> String.downcase()
+    |> underscore()
   end
 
   @doc """
@@ -43,4 +43,30 @@ defmodule Cereal.Utils do
 
   defp deep_merge_relationship_paths(left, right), do: Keyword.merge(left, right, &deep_merge_relationship_paths/3)
   defp deep_merge_relationship_paths(_, left, right), do: deep_merge_relationship_paths(left, right)
+
+
+  @doc false
+  def underscore(""), do: ""
+  def underscore(<<h, t :: binary>>) do
+    <<to_lower_char(h)>> <> do_underscore(t, h)
+  end
+
+  defp do_underscore(<<h, t, rest :: binary>>, _) when h in ?A..?Z and not (t in ?A..?Z or t == ?.) do
+    <<?_, to_lower_char(h), t>> <> do_underscore(rest, t)
+  end
+  defp do_underscore(<<h, t :: binary>>, prev) when h in ?A..?Z and not prev in ?A..?Z do
+    <<?_, to_lower_char(h)>> <> do_underscore(t, h)
+  end
+  defp do_underscore(<<?., t :: binary>>, _) do
+    <<?/>> <> underscore(t)
+  end
+  defp do_underscore(<<h, t :: binary>>, _) do
+    <<to_lower_char(h)>> <> do_underscore(t, h)
+  end
+  defp do_underscore(<<>>, _) do
+    <<>>
+  end
+
+  defp to_lower_char(char) when char in ?A..?Z, do: char + 32
+  defp to_lower_char(char), do: char
 end