fix issue with dual

Project.toml

@@ -10,6 +10,7 @@ DataStructures = "864edb3b-99cc-5e75-8d2d-829cb0a9cfe8"
 Distributed = "8ba89e20-285c-5b6f-9357-94700520ee1b"
 GraphOptInterface = "2c3d953e-07eb-42f7-9900-e75950e41d88"
 Graphs = "86223c79-3864-5bf0-83f7-82e725a168b6"
+Ipopt = "b6b21f68-93f8-5de0-b562-5493be1d77c9"
 JuMP = "4076af6c-e467-56ae-b986-b466b2749572"
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"
 MathOptInterface = "b8f27783-ece8-5eb3-8dc8-9495eed66fee"

src/core_types.jl

@@ -85,3 +85,11 @@ const OptiElement = Union{OptiNode{<:AbstractOptiGraph},OptiEdge{<:AbstractOptiG
 const OptiObject = Union{
     OptiNode{<:AbstractOptiGraph},OptiEdge{<:AbstractOptiGraph},OptiGraph
 }
+
+const NodeConstraintRef = JuMP.ConstraintRef{
+    OptiNode{GT},MOI.ConstraintIndex{FT,ST},<:JuMP.AbstractShape
+} where {GT<:AbstractOptiGraph,FT<:MOI.AbstractFunction,ST<:MOI.AbstractSet}
+
+const EdgeConstraintRef = JuMP.ConstraintRef{
+    OptiEdge{GT},MOI.ConstraintIndex{FT,ST},<:JuMP.AbstractShape
+} where {GT<:AbstractOptiGraph,FT<:MOI.AbstractFunction,ST<:MOI.AbstractSet}

src/optiedge.jl

@@ -15,6 +15,10 @@ function Base.getindex(edge::OptiEdge, name::Symbol)
     return edge.source_graph.edge_obj_dict[t]
 end
 
+const EdgeConstraintRef = JuMP.ConstraintRef{
+    OptiEdge{GT},MOI.ConstraintIndex{FT,ST},<:JuMP.AbstractShape
+} where {GT<:AbstractOptiGraph,FT<:MOI.AbstractFunction,ST<:MOI.AbstractSet}
+
 """
     graph_backend(edge::OptiEdge)
 
@@ -110,3 +114,13 @@ end
 function JuMP.is_valid(edge::OptiEdge, cref::ConstraintRef)
     return edge === JuMP.owner_model(cref) && MOI.is_valid(graph_backend(edge), cref)
 end
+
+"""
+    JuMP.dual(cref::EdgeConstraintRef; result::Int=1)
+
+Return the dual for an `EdgeConstraintRef`. This returns the dual for the source graph that
+corresponds to the constraint reference.
+"""
+function JuMP.dual(cref::EdgeConstraintRef; result::Int=1)
+    return MOI.get(graph_backend(cref.model), MOI.ConstraintDual(result), cref)
+end

src/optigraph.jl

@@ -858,6 +858,28 @@ function JuMP.value(graph::OptiGraph, expr::GenericNonlinearExpr; result::Int=1)
     end
 end
 
+"""
+    JuMP.dual(graph::OptiGraph, cref::NodeConstraintRef; result::Int=1)
+
+Return the dual value of `cref` in `graph`. Note that this value is specific to 
+the optimizer solution to the `graph`. The `cref` can have different values for 
+different optigraphs it is contained in.
+"""
+function JuMP.dual(graph::OptiGraph, cref::NodeConstraintRef; result::Int=1)
+    return MOI.get(graph_backend(graph), MOI.ConstraintDual(result), cref)
+end
+
+"""
+    JuMP.dual(graph::OptiGraph, cref::EdgeConstraintRef; result::Int=1)
+
+Return the dual value of `cref` in `graph`. Note that this value is specific to 
+the optimizer solution to the `graph`. The `cref` can have different values for 
+different optigraphs it is contained in.
+"""
+function JuMP.dual(graph::OptiGraph, cref::EdgeConstraintRef; result::Int=1)
+    return MOI.get(graph_backend(graph), MOI.ConstraintDual(result), cref)
+end
+
 ### Constraints
 
 """

src/optinode.jl

@@ -118,6 +118,18 @@ function JuMP.delete(node::OptiNode, cref::ConstraintRef)
     return nothing
 end
 
+### Duals
+
+"""
+    JuMP.dual(cref::NodeConstraintRef; result::Int=1)
+
+Return the dual for a `NodeConstraintRef`. This returns the dual for the source graph that
+corresponds to the constraint reference.
+"""
+function JuMP.dual(cref::NodeConstraintRef; result::Int=1)
+    return MOI.get(graph_backend(cref.model), MOI.ConstraintDual(result), cref)
+end
+
 ### Constraints
 
 """

test/test_optigraph.jl

@@ -32,6 +32,11 @@ function test_simple_graph()
     @test JuMP.dual_status(graph) == MOI.FEASIBLE_POINT
     @test JuMP.result_count(graph) == 1
     @test JuMP.raw_status(graph) == "kHighsModelStatusOptimal"
+
+    constraints = all_constraints(graph)
+    @test JuMP.dual(constraints[1]) == 1.0
+    @test JuMP.dual(constraints[2]) == 0.0
+    @test JuMP.dual(constraints[3]) == -2.0
 end
 
 function _create_test_nonlinear_optigraph()