Improve DB::MappingException usage (#129)

spec/serializable_spec.cr

@@ -105,19 +105,19 @@ describe "DB::Serializable" do
   end
 
   it "should fail to initialize a simple model if types do not match" do
-    expect_raises ArgumentError do
+    expect_raises DB::MappingException, "Invalid Int32: b\n  deserializing SimpleModel#c0" do
       from_dummy("b,a", SimpleModel)
     end
   end
 
   it "should fail to initialize a simple model if there is a missing column" do
-    expect_raises DB::MappingException do
+    expect_raises DB::MappingException, "Missing column c1\n  deserializing SimpleModel#c1" do
       from_dummy("1", SimpleModel)
     end
   end
 
   it "should fail to initialize a simple model if there is an unexpected column" do
-    expect_raises DB::MappingException do
+    expect_raises DB::MappingException, "Unknown column: c2\n  deserializing SimpleModel" do
       from_dummy("1,a,b", SimpleModel)
     end
   end

src/db/error.cr

@@ -6,6 +6,19 @@ module DB
   end
 
   class MappingException < Error
+    getter klass
+    getter property
+
+    def initialize(message, @klass : String, @property : String? = nil, cause : Exception? = nil)
+      message = String.build do |io|
+        io << message
+        io << "\n  deserializing " << @klass
+        if property = @property
+          io << "#" << property
+        end
+      end
+      super(message, cause: cause)
+    end
   end
 
   class PoolTimeout < Error

src/db/mapping.cr

@@ -117,7 +117,7 @@ module DB
           {% end %}
           else
             {% if strict %}
-              raise ::DB::MappingException.new("unknown result set attribute: #{col_name}")
+              raise ::DB::MappingException.new("unknown result set attribute: #{col_name}", self.class.to_s)
             {% else %}
               %rs.read
             {% end %}
@@ -127,7 +127,7 @@ module DB
       {% for key, value in properties %}
         {% unless value[:nilable] || value[:default] != nil %}
           if %var{key.id}.is_a?(Nil) && !%found{key.id}
-            raise ::DB::MappingException.new("missing result set attribute: {{(value[:key] || key).id}}")
+            raise ::DB::MappingException.new("missing result set attribute: {{(value[:key] || key).id}}", self.class.to_s)
           end
         {% end %}
       {% end %}

src/db/serializable.cr

@@ -129,14 +129,18 @@ module DB
             {% for name, value in properties %}
               when {{value[:key]}}
                 %found{name} = true
-                %var{name} =
-                  {% if value[:converter] %}
-                    {{value[:converter]}}.from_rs(rs)
-                  {% elsif value[:nilable] || value[:default] != nil %}
-                    rs.read(::Union({{value[:type]}} | Nil))
-                  {% else %}
-                    rs.read({{value[:type]}})
-                  {% end %}
+                begin
+                  %var{name} =
+                    {% if value[:converter] %}
+                      {{value[:converter]}}.from_rs(rs)
+                    {% elsif value[:nilable] || value[:default] != nil %}
+                      rs.read(::Union({{value[:type]}} | Nil))
+                    {% else %}
+                      rs.read({{value[:type]}})
+                    {% end %}
+                rescue exc
+                  ::raise ::DB::MappingException.new(exc.message, self.class.to_s, {{name.stringify}}, cause: exc)
+                end
             {% end %}
           else
             rs.read # Advance set, but discard result
@@ -146,8 +150,8 @@ module DB
 
         {% for key, value in properties %}
           {% unless value[:nilable] || value[:default] != nil %}
-            if %var{key}.is_a?(Nil) && !%found{key}
-              raise ::DB::MappingException.new("missing result set attribute: {{(value[:key] || key).id}}")
+            if %var{key}.nil? && !%found{key}
+              ::raise ::DB::MappingException.new("Missing column {{value[:key].id}}", self.class.to_s, {{key.stringify}})
             end
           {% end %}
         {% end %}
@@ -169,7 +173,7 @@ module DB
     end
 
     protected def on_unknown_db_column(col_name)
-      raise ::DB::MappingException.new("unknown result set attribute: #{col_name}")
+      ::raise ::DB::MappingException.new("Unknown column: #{col_name}", self.class.to_s)
     end
 
     module NonStrict