JuMP and Gurobi.jl

# First comment out lines https://github.com/JuliaOpt/Gurobi.jl/blob/master/src/GurobiSolverInterface.jl#L246-L254
using Gurobi,JuMP

# Create the JuMP Model:
# min 0
# s.t.
#     x >= 1
#     x <= 3
m=Model(solver=GurobiSolver())
@variable(m, x)
@constraints(m, begin
    x >= 1
    x <= 3
end)

# initialise Gurobi Model
JuMP.build(m)

# Change Gurobi model to
# min -y
# s.t.
#     x - y >= 1
#     x + y <= 3
#     y ∈ [0, 1]
# has optimal solution (x,y) = (2, 1), obj=-1
MathProgBase.addvar!(internalmodel(m), [1, 2], [-1.0, 1.0], 0, 1, -1)

Gurobi.updatemodel!(internalmodel(m))

@assert Gurobi.getobj(internalmodel(m)) == [0.0, -1.0]
@assert Gurobi.getsense(internalmodel(m)) == :Min
@assert Gurobi.num_vars(internalmodel(m).inner) == 2

@assert solve(m) == :Optimal

@assert getobjectivesense(m) == :Min

@assert Gurobi.getobj(internalmodel(m)) == [0.0, -1.0] # fails: reports [0.0, 3.19e-308]
@assert getobjectivevalue(m) == -1                     # fails: reports 0.0