Add support for nillables with `T | Nil` syntax.

spec/mapping_spec.cr

@@ -36,6 +36,13 @@ class MappingWithNilTypes
   })
 end
 
+class MappingWithNilUnionTypes
+  DB.mapping({
+    c0: {type: Int32 | Nil, default: 10},
+    c1: Nil | String,
+  })
+end
+
 class MappingWithKeys
   DB.mapping({
     foo: {type: Int32, key: "c0"},
@@ -117,10 +124,12 @@ describe "DB.mapping" do
 
   it "should initialize a mapping with nilable types if columns are missing" do
     expect_mapping("1", MappingWithNilTypes, {c0: 1, c1: nil})
+    expect_mapping("1", MappingWithNilUnionTypes, {c0: 1, c1: nil})
   end
 
   it "should initialize a mapping with nilable types ignoring default value if NULL" do
     expect_mapping("NULL,a", MappingWithNilTypes, {c0: nil, c1: "a"})
+    expect_mapping("NULL,a", MappingWithNilUnionTypes, {c0: nil, c1: "a"})
   end
 
   it "should initialize a mapping with different keys" do

src/db/mapping.cr

@@ -66,6 +66,11 @@ module DB
 
     {% for key, value in properties %}
       {% value[:nilable] = true if value[:type].is_a?(Generic) && value[:type].type_vars.map(&.resolve).includes?(Nil) %}
+
+      {% if value[:type].is_a?(Call) && value[:type].name == "|" &&
+              (value[:type].receiver.resolve == Nil || value[:type].args.map(&.resolve).any?(&.==(Nil))) %}
+        {% value[:nilable] = true %}
+      {% end %}
     {% end %}
 
     {% for key, value in properties %}
@@ -103,7 +108,7 @@ module DB
                 {% if value[:converter] %}
                   {{value[:converter]}}.from_rs(%rs)
                 {% elsif value[:nilable] || value[:default] != nil %}
-                  %rs.read(Union({{value[:type]}} | Nil))
+                  %rs.read(::Union({{value[:type]}} | Nil))
                 {% else %}
                   %rs.read({{value[:type]}})
                 {% end %}