Deletion of constraint leads to information loss

[dhendryc]

Hi,

it seems that deleting one constraint erases more information than it should.
Below is a MWE. With HiGHS everything runs as expected, but we get the following error with SCIP

Add equality constraint...
┌ Error: SCIP test failed with error:
│   exception =
│    KeyError: key MOI.VariableIndex(1) not found
│    Stacktrace:
│     [1] getindex(h::Dict{MathOptInterface.VariableIndex, MathOptInterface.Interval{Float64}}, key::MathOptInterface.VariableIndex)
│       @ Base ./dict.jl:477
│     [2] add_constraint(o::SCIP.Optimizer, vi::MathOptInterface.VariableIndex, set::MathOptInterface.EqualTo{Float64})
│       @ SCIP ~/.julia/packages/SCIP/1OzGv/src/MOI_wrapper/variable.jl:322
│     [3] build_and_modify_model(optimizer_constructor::Type{SCIP.Optimizer}, optimizer_name::String)
│       @ Main ~/Documents/research_projects/minimal_constraint_deletion_example.jl:55
│     [4] top-level scope
│       @ ~/Documents/research_projects/minimal_constraint_deletion_example.jl:77
│     [5] include(mod::Module, _path::String)
│       @ Base ./Base.jl:557
│     [6] exec_options(opts::Base.JLOptions)
│       @ Base ./client.jl:323
│     [7] _start()
│       @ Base ./client.jl:531
└ @ Main ~/Documents/research_projects/minimal_constraint_deletion_example.jl:80

while trying to add the equal to constraint.
For some background, we want to fix specific (less than, greater than) constraints to equality during the solve but they might not necessarily be bound constraints. Hence, we are reading out the function format instead creating a new function.

using MathOptInterface
using SCIP
using HiGHS
const MOI = MathOptInterface

function build_and_modify_model(optimizer_constructor, optimizer_name)
    println("=" ^ 50)
    println("Testing with $optimizer_name")
    println("=" ^ 50)
    
    # Build the model
    o = optimizer_constructor()
    MOI.set(o, MOI.Silent(), true)
    MOI.empty!(o)
    
    n = 3  # Number of variables
    x = MOI.add_variables(o, n)
    
    # Add constraints
    greater_than_constraints = []
    less_than_constraints = []
    binary_constraints = []
    
    for (i, xi) in enumerate(x)
        # Add greater than constraint
        gt_constraint = MOI.add_constraint(o, xi, MOI.GreaterThan(0.0))
        push!(greater_than_constraints, gt_constraint)
        
        # Add less than constraint
        lt_constraint = MOI.add_constraint(o, xi, MOI.LessThan(1.0))
        push!(less_than_constraints, lt_constraint)
        
        # Add binary constraint
        bin_constraint = MOI.add_constraint(o, xi, MOI.ZeroOne())
        push!(binary_constraints, bin_constraint)
    end

    func = MOI.get(o, MOI.ConstraintFunction(), greater_than_constraints[1])
    set = MOI.get(o, MOI.ConstraintSet(), greater_than_constraints[1])
    
    println("Model after building:")
    print(o)
    
    # Delete the first greater than constraint
    println("\nDeleting first less than constraint...")
    MOI.delete(o, less_than_constraints[1])
    

    println("\nDelete first greater than constraint...")
    check_cidx = MOI.ConstraintIndex{MOI.VariableIndex,MOI.GreaterThan{Float64}}(greater_than_constraints[1].value)
    MOI.delete(o, check_cidx)
    

    println("\nAdd equality constraint...")
    MOI.add_constraint(o, func, MOI.EqualTo(set.lower))
    
    println("\nModel after adding equality constraint:")
    print(o)
    
    return o
end

# Test with HiGHS
println("Starting HiGHS test...")
try
    o = build_and_modify_model(HiGHS.Optimizer, "HiGHS")
    println("HiGHS test completed successfully!")
catch e
    @error "HiGHS test failed with error:" exception=(e, catch_backtrace())
end

println("\n" * "=" ^ 80 * "\n")

# Test with SCIP
println("Starting SCIP test...")
try
    o = build_and_modify_model(SCIP.Optimizer, "SCIP")
    println("SCIP test completed successfully!")
catch e
    @error "SCIP test failed with error:" exception=(e, catch_backtrace())
end

[odow]

Simplified to

julia> import MathOptInterface as MOI

julia> using SCIP

julia> function main()
           model = SCIP.Optimizer()
           x = MOI.add_variable(model)
           c_gt = MOI.add_constraint(model, x, MOI.GreaterThan(0.0))
           c_lt = MOI.add_constraint(model, x, MOI.LessThan(1.0))
           c_bin = MOI.add_constraint(model, x, MOI.ZeroOne())
           MOI.delete(model, c_lt)
           MOI.delete(model, c_gt)
           MOI.add_constraint(model, x, MOI.EqualTo(0.0))
           return
       end
main (generic function with 1 method)

julia> main()
ERROR: KeyError: key MOI.VariableIndex(1) not found
Stacktrace:
 [1] getindex
   @ ./dict.jl:498 [inlined]
 [2] add_constraint(o::SCIP.Optimizer, vi::MathOptInterface.VariableIndex, set::MathOptInterface.EqualTo{Float64})
   @ SCIP ~/.julia/dev/SCIP/src/MOI_wrapper/variable.jl:322
 [3] main()
   @ Main ./REPL[54]:9
 [4] top-level scope
   @ REPL[55]:1