correctly handle objective constant with gurobi

Author: peterlietz

mip/gurobi.py

@@ -525,6 +525,7 @@ def get_objective(self) -> LinExpr:
             for i in range(self.num_cols())
             if abs(obj[i] > 1e-20)
         )
+        obj_expr.const = self.get_objective_const()
         obj_expr.sense = self.get_objective_sense
         return obj_expr
 
@@ -1263,6 +1264,7 @@ def set_dbl_attr(self, name: str, value: float):
             raise ParameterNotAvailable(
                 "Error modifying double attribute {} to {}".format(name, value)
             )
+        GRBupdatemodel(self._model)
 
     def get_int_attr(self, name: str) -> int:
         res = ffi.new("int *")

test/mip_test.py

@@ -547,6 +547,21 @@ def test_var_by_name_rhs(self, solver):
         v = model.var_by_name("x({},{})".format(0, 0))
         assert v is not None
 
+    @pytest.mark.parametrize("solver", SOLVERS)
+    def test_obj_const1(self, solver: str):
+        n, model = self.build_model(solver)
+
+        model.objective = 1
+        e = model.objective
+        assert e.const == 1
+
+    @pytest.mark.parametrize("solver", SOLVERS)
+    def test_obj_const2(self, solver: str):
+        n, model = self.build_model(solver)
+
+        model.objective = 1
+        assert model.objective_const == 1
+
 
 @pytest.mark.parametrize("val", range(1, 4))
 @pytest.mark.parametrize("solver", SOLVERS)