diff --git a/lib/jsonapi/plugs/query_parser.ex b/lib/jsonapi/plugs/query_parser.ex
index 5e27b8bf..a9153028 100644
--- a/lib/jsonapi/plugs/query_parser.ex
+++ b/lib/jsonapi/plugs/query_parser.ex
@@ -253,7 +253,7 @@ defmodule JSONAPI.QueryParser do
     if JString.field_transformation() == :underscore do
       fields
     else
-      JString.underscore(fields)
+      JString.expand_fields(fields, &JString.underscore/1)
     end
   end
 end
diff --git a/lib/jsonapi/plugs/underscore_parameters.ex b/lib/jsonapi/plugs/underscore_parameters.ex
index 83c926f5..7e685b2a 100644
--- a/lib/jsonapi/plugs/underscore_parameters.ex
+++ b/lib/jsonapi/plugs/underscore_parameters.ex
@@ -33,7 +33,7 @@ defmodule JSONAPI.UnderscoreParameters do
 
   import Plug.Conn
 
-  import JSONAPI.Utils.String, only: [underscore: 1]
+  alias JSONAPI.Utils.String, as: JString
 
   @doc false
   def init(_opts) do
@@ -44,7 +44,7 @@ defmodule JSONAPI.UnderscoreParameters do
     content_type = get_req_header(conn, "content-type")
 
     if JSONAPI.mime_type() in content_type do
-      new_params = underscore(params)
+      new_params = JString.expand_fields(params, &JString.underscore/1)
 
       Map.put(conn, :params, new_params)
     else
diff --git a/lib/jsonapi/serializer.ex b/lib/jsonapi/serializer.ex
index 4e9009dd..4c92d1da 100644
--- a/lib/jsonapi/serializer.ex
+++ b/lib/jsonapi/serializer.ex
@@ -231,8 +231,8 @@ defmodule JSONAPI.Serializer do
 
   defp transform_fields(fields) do
     case JString.field_transformation() do
-      :camelize -> JString.camelize(fields)
-      :dasherize -> JString.dasherize(fields)
+      :camelize -> JString.expand_fields(fields, &JString.camelize/1)
+      :dasherize -> JString.expand_fields(fields, &JString.dasherize/1)
       _ -> fields
     end
   end
diff --git a/lib/jsonapi/utils/string.ex b/lib/jsonapi/utils/string.ex
index a74636d2..6319b145 100644
--- a/lib/jsonapi/utils/string.ex
+++ b/lib/jsonapi/utils/string.ex
@@ -17,56 +17,26 @@ defmodule JSONAPI.Utils.String do
       iex> underscore("top-posts")
       "top_posts"
 
+      iex> underscore(:top_posts)
+      "top_posts"
+
       iex> underscore("-top-posts")
       "-top_posts"
 
       iex> underscore("-top--posts-")
       "-top--posts-"
 
-      iex> underscore(%{"foo-bar" => "baz"})
-      %{"foo_bar" => "baz"}
-
-      iex> underscore({"foo-bar", "dollar-sol"})
-      {"foo_bar", "dollar-sol"}
-
-      iex> underscore({"foo-bar", %{"a-d" => "z-8"}})
-      {"foo_bar", %{"a_d" => "z-8"}}
-
-      iex> underscore(%{"f-b" => %{"a-d" => "z"}, "c-d" => "e"})
-      %{"f_b" => %{"a_d" => "z"}, "c_d" => "e"}
-
-      iex> underscore(:"foo-bar")
-      :foo_bar
-
-      iex> underscore(%{"f-b" => "a-d"})
-      %{"f_b" => "a-d"}
-
   """
+  @spec underscore(String.t()) :: String.t()
   def underscore(value) when is_binary(value) do
     String.replace(value, ~r/([a-zA-Z0-9])-([a-zA-Z0-9])/, "\\1_\\2")
   end
 
-  def underscore(map) when is_map(map) do
-    Enum.into(map, %{}, &underscore/1)
-  end
-
-  def underscore({key, value}) when is_map(value) do
-    {underscore(key), underscore(value)}
-  end
-
-  def underscore({key, value}) do
-    {underscore(key), value}
-  end
-
+  @spec underscore(atom) :: atom
   def underscore(value) when is_atom(value) do
     value
     |> to_string()
     |> underscore()
-    |> String.to_atom()
-  end
-
-  def underscore(value) do
-    value
   end
 
   @doc """
@@ -86,32 +56,18 @@ defmodule JSONAPI.Utils.String do
       "_top__posts_"
 
   """
+  @spec dasherize(atom) :: atom
   def dasherize(value) when is_atom(value) do
     value
     |> to_string()
     |> dasherize()
   end
 
+  @spec dasherize(String.t()) :: String.t()
   def dasherize(value) when is_binary(value) do
     String.replace(value, ~r/([a-zA-Z0-9])_([a-zA-Z0-9])/, "\\1-\\2")
   end
 
-  def dasherize(%{__struct__: _} = value) when is_map(value) do
-    value
-  end
-
-  def dasherize(value) when is_map(value) do
-    Enum.into(value, %{}, &dasherize/1)
-  end
-
-  def dasherize({key, value}) do
-    if is_map(value) do
-      {dasherize(key), dasherize(value)}
-    else
-      {dasherize(key), value}
-    end
-  end
-
   @doc """
   Replace underscores or dashes between words in `value` with camelCasing
 
@@ -122,6 +78,9 @@ defmodule JSONAPI.Utils.String do
       iex> camelize("top_posts")
       "topPosts"
 
+      iex> camelize(:top_posts)
+      "topPosts"
+
       iex> camelize("_top_posts")
       "_topPosts"
 
@@ -129,12 +88,14 @@ defmodule JSONAPI.Utils.String do
       "_top__posts_"
 
   """
+  @spec camelize(atom) :: atom
   def camelize(value) when is_atom(value) do
     value
     |> to_string()
     |> camelize()
   end
 
+  @spec camelize(String.t()) :: String.t()
   def camelize(value) when is_binary(value) do
     with words <-
            Regex.split(
@@ -154,20 +115,68 @@ defmodule JSONAPI.Utils.String do
     [String.capitalize(h)] ++ camelize_list(t)
   end
 
-  def camelize(%{__struct__: _} = value) when is_map(value) do
-    value
+  @doc """
+
+  ## Examples
+
+      iex> expand_fields(%{"foo-bar" => "baz"}, &underscore/1)
+      %{"foo_bar" => "baz"}
+
+      iex> expand_fields(%{"foo_bar" => "baz"}, &dasherize/1)
+      %{"foo-bar" => "baz"}
+
+      iex> expand_fields(%{"foo-bar" => "baz"}, &camelize/1)
+      %{"fooBar" => "baz"}
+
+      iex> expand_fields({"foo-bar", "dollar-sol"}, &underscore/1)
+      {"foo_bar", "dollar-sol"}
+
+      iex> expand_fields({"foo-bar", %{"a-d" => "z-8"}}, &underscore/1)
+      {"foo_bar", %{"a_d" => "z-8"}}
+
+      iex> expand_fields(%{"f-b" => %{"a-d" => "z"}, "c-d" => "e"}, &underscore/1)
+      %{"f_b" => %{"a_d" => "z"}, "c_d" => "e"}
+
+      iex> expand_fields(%{"f-b" => %{"a-d" => %{"z-w" => "z"}}, "c-d" => "e"}, &underscore/1)
+      %{"f_b" => %{"a_d" => %{"z_w" => "z"}}, "c_d" => "e"}
+
+      iex> expand_fields(:"foo-bar", &underscore/1)
+      "foo_bar"
+
+      iex> expand_fields(:foo_bar, &dasherize/1)
+      "foo-bar"
+
+      iex> expand_fields(:"foo-bar", &camelize/1)
+      "fooBar"
+
+      iex> expand_fields(%{"f-b" => "a-d"}, &underscore/1)
+      %{"f_b" => "a-d"}
+
+      iex> expand_fields(%{"inserted-at" => ~N[2019-01-17 03:27:24.776957]}, &underscore/1)
+      %{"inserted_at" => ~N[2019-01-17 03:27:24.776957]}
+
+  """
+  @spec expand_fields(map, function) :: map
+  def expand_fields(%{__struct__: _} = value, _fun), do: value
+
+  @spec expand_fields(map, function) :: map
+  def expand_fields(map, fun) when is_map(map) do
+    Enum.into(map, %{}, &expand_fields(&1, fun))
   end
 
-  def camelize(value) when is_map(value) do
-    Enum.into(value, %{}, &camelize/1)
+  @spec expand_fields(tuple, function) :: tuple
+  def expand_fields({key, value}, fun) when is_map(value) do
+    {fun.(key), expand_fields(value, fun)}
   end
 
-  def camelize({key, value}) do
-    if is_map(value) do
-      {camelize(key), camelize(value)}
-    else
-      {camelize(key), value}
-    end
+  @spec expand_fields(tuple, function) :: tuple
+  def expand_fields({key, value}, fun) do
+    {fun.(key), value}
+  end
+
+  @spec expand_fields(any, function) :: any
+  def expand_fields(value, fun) do
+    fun.(value)
   end
 
   defp normalized_underscore_to_dash_config(value) when is_boolean(value) do