Fix broken test for utils.solver with symengine (#1340)

* repair broken test

* add RN

* resolve review comments

Author: cdiener

release-notes/next-release.md

@@ -1,4 +1,4 @@
-# Release notes for cobrapy x.y.z
+# Release notes for cobrapy 0.27.0
 
 ## New features
 
@@ -12,6 +12,8 @@ has been removed.
 `loopless_solution` now fixes the objective to its optimum as in the
 originally published method and returns the objective value in the solution object.
 
+Repair a broken test for `fix_objective_as_constraint`.
+
 ## Other
 
 Backwards compatibility for pickled models has been improved.

tests/test_util/test_solver.py

@@ -118,15 +118,18 @@ def test_absolute_expression(model: "Model") -> None:
 def test_fix_objective_as_constraint(solver: str, model: "Model") -> None:
     """Test fixing present objective as a constraint."""
     model.solver = solver
+    opt = model.slim_optimize()
     with model as m:
-        su.fix_objective_as_constraint(model, 1.0)
-        constraint_name = m.constraints[-1]
-        assert abs(m.constraints[-1].expression - m.objective.expression) < 1e-6
-    assert constraint_name not in m.constraints
-    su.fix_objective_as_constraint(model)
-    constraint_name = model.constraints[-1]
-    assert abs(model.constraints[-1].expression - model.objective.expression) < 1e-6
-    assert constraint_name in model.constraints
+        su.fix_objective_as_constraint(model, 1.0, name="fixed")
+        assert (m.constraints.fixed.expression - m.objective.expression).simplify() == 0
+        assert m.constraints.fixed.lb == pytest.approx(opt)
+    assert "fixed" not in m.constraints
+    su.fix_objective_as_constraint(model, name="fixed")
+    assert (
+        model.constraints.fixed.expression - model.objective.expression
+    ).simplify() == 0
+    assert m.constraints.fixed.lb == pytest.approx(opt)
+    assert "fixed" in model.constraints
 
 
 @pytest.mark.parametrize("solver", optlang_solvers)