From b3fa54e6714caecde5112c1ae0021fdfe06f1107 Mon Sep 17 00:00:00 2001
From: Oscar Dowson <o.dowson@gmail.com>
Date: Fri, 15 May 2026 13:43:10 +1200
Subject: [PATCH] Fix writing to a MOI.FileFormats.MOF file

This is necessary for Sienna. Rather than do something custom, we just
force-add the constraint.
---
 src/MathOptLazy.jl |  7 +++++++
 test/runtests.jl   | 12 ++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/src/MathOptLazy.jl b/src/MathOptLazy.jl
index c48e128..509f3d7 100644
--- a/src/MathOptLazy.jl
+++ b/src/MathOptLazy.jl
@@ -23,6 +23,13 @@ function MOI.Utilities.shift_constant(set::LazyScalarSet, constant)
     return LazyScalarSet(MOI.Utilities.shift_constant(set.set, constant))
 end
 
+function MOI.FileFormats.MOF.moi_to_object(
+    set::LazyScalarSet,
+    x::Dict{MOI.VariableIndex,String},
+)
+    return MOI.FileFormats.MOF.moi_to_object(set.set, x)
+end
+
 """
     Lazy(; lazy::Bool = true)
 
diff --git a/test/runtests.jl b/test/runtests.jl
index d9e4fd8..bc406d1 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -162,6 +162,18 @@ function test_basic_scalaraffinefunction_lessthan()
     return
 end
 
+function test_writing_mof_file()
+    src = MathOptLazy.Optimizer(HiGHS.Optimizer)
+    x = MOI.add_variable(src)
+    c = MOI.add_constraint(src, x, MathOptLazy.LazyScalarSet(MOI.ZeroOne()))
+    dest = MOI.FileFormats.MOF.Model()
+    _ = MOI.copy_to(dest, src)
+    contents = sprint(write, dest)
+    @test occursin("ZeroOne", contents)
+    @test !occursin("LazyScalarSet", contents)
+    return
+end
+
 end  # TestMathOptLazy
 
 TestMathOptLazy.runtests()