be smarter when assembling the constraints expression

CHANGELOG.rst

@@ -3,6 +3,12 @@
 Next Release
 -----
 
+1.9.1
+-----
+* expressions for Constraint in the matrix interface are now assembled from only non-zero
+  coefficients, speeding up JSON conversion as a side effect
+* fixed a typo in the matrix interface
+
 1.9.0
 -----
 * enable Python 3.14

src/optlang/matrix_interface.py

@@ -452,7 +452,11 @@ def _get_expression(self):
         if self.problem is not None:
             variables = self.problem._variables
             all_coefs = self.problem.problem.constraint_coefs
-            coefs = [(v, all_coefs.get((self.name, v.name), 0.0)) for v in variables]
+            coefs = [
+                (variables[vname], coef)
+                for (cname, vname), coef in all_coefs.items()
+                if cname == self.name
+            ]
             expression = add([mul((symbolics.Real(co), v)) for (v, co) in coefs])
             self._expression = expression
         return self._expression
@@ -823,15 +827,15 @@ def objective(self, value):
         (
             offset,
             linear_coefficients,
-            quadratic_coeffients,
+            quadratic_coefficients,
         ) = parse_optimization_expression(value, quadratic=True, expression=expression)
         self._objective_offset = offset
         if linear_coefficients:
             self.problem.obj_linear_coefs = {
                 v.name: float(c) for v, c in linear_coefficients.items()
             }
 
-        for key, coef in quadratic_coeffients.items():
+        for key, coef in quadratic_coefficients.items():
             if len(key) == 1:
                 var = next(iter(key))
                 self.problem.obj_quadratic_coefs[(var.name, var.name)] = float(coef)