Fix namespaced vs top-level type-name collisions

spec/serializable_spec.cr

@@ -104,6 +104,23 @@ struct ModelWithEnum
   end
 end
 
+# Ensure types that happen to have the same name as types within the `DB`
+# namespace don't clash here
+struct Transaction
+  include DB::Serializable
+
+  @[DB::Field(key: "c0")]
+  getter id : Int32
+  @[DB::Field(key: "c1")]
+  getter status : Status
+
+  enum Status
+    Pending
+    Complete
+    Canceled
+  end
+end
+
 macro from_dummy(query, type)
   with_dummy do |db|
     rs = db.query({{ query }})
@@ -206,6 +223,13 @@ describe "DB::Serializable" do
     end
   end
 
+  it "should compile when top-level names collide with DB-namespaced names" do
+    expect_model("1,Pending", Transaction, {
+      id:     1,
+      status: Transaction::Status::Pending,
+    })
+  end
+
   it "should initialize multiple instances from a single resultset" do
     with_dummy do |db|
       db.query("1,a 2,b") do |rs|

src/db/serializable.cr

@@ -109,7 +109,11 @@ module DB
           {% unless ann && ann[:ignore] %}
             {%
               properties[ivar.id] = {
-                type:      ivar.type,
+                type: if ivar.type.union?
+                  "Union(#{ivar.type.union_types.map { |t| "::#{t}".id }.join(" | ").id})".id
+                else
+                  "::#{ivar.type}".id
+                end,
                 key:       ((ann && ann[:key]) || ivar).id.stringify,
                 default:   ivar.default_value,
                 nilable:   ivar.type.nilable?,