Allow nilable types in DB.mapping

Previously, using a Nilable type in DB.mapping instead of nilable: true would
fail with a nil assertion error.

spec/mapping_spec.cr

@@ -29,6 +29,13 @@ class MappingWithNilables
   })
 end
 
+class MappingWithNilTypes
+  DB.mapping({
+    c0: {type: Int32?, default: 10},
+    c1: String?,
+  })
+end
+
 class MappingWithKeys
   DB.mapping({
     foo: {type: Int32, key: "c0"},
@@ -100,14 +107,22 @@ describe "DB.mapping" do
     expect_mapping("1,NULL", MappingWithDefaults, {c0: 1, c1: "c"})
   end
 
-  it "should initialize a mapping with nils if columns are missing" do
+  it "should initialize a mapping with nilable set if columns are missing" do
     expect_mapping("1", MappingWithNilables, {c0: 1, c1: nil})
   end
 
-  it "should initialize a mapping with nils ignoring default value is type is nilable" do
+  it "should initialize a mapping with nilable set ignoring default value if NULL" do
     expect_mapping("NULL,a", MappingWithNilables, {c0: nil, c1: "a"})
   end
 
+  it "should initialize a mapping with nilable types if columns are missing" do
+    expect_mapping("1", MappingWithNilTypes, {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"})
+  end
+
   it "should initialize a mapping with different keys" do
     expect_mapping("1,a", MappingWithKeys, {foo: 1, bar: "a"})
   end

src/db/mapping.cr

@@ -64,6 +64,10 @@ module DB
       {% properties[key] = {type: value} unless value.is_a?(HashLiteral) || value.is_a?(NamedTupleLiteral) %}
     {% end %}
 
+    {% for key, value in properties %}
+      {% value[:nilable] = true if value[:type].is_a?(Generic) && value[:type].type_vars.map(&.resolve).includes?(Nil) %}
+    {% end %}
+
     {% for key, value in properties %}
       @{{key.id}} : {{value[:type]}} {{ (value[:nilable] ? "?" : "").id }}
 
@@ -131,7 +135,7 @@ module DB
         {% elsif value[:default] != nil %}
           @{{key.id}} = %var{key.id}.is_a?(Nil) ? {{value[:default]}} : %var{key.id}
         {% else %}
-          @{{key.id}} = %var{key.id}.not_nil!
+          @{{key.id}} = %var{key.id}.as({{value[:type]}})
         {% end %}
       {% end %}
     end