Printing models with parameters

[joaquimg]

Consider:

model = Model()
@variable(model, y == 18)
@variable(model, p in Parameter(1.3))
@constraint(model, c1, 2y + 3p >= 0)
@objective(model, Min, 5y + 6p)

write_to_file(model, "test_param.mps")

print(read("test_param.mps", String))

The resulting file is:

NAME
ROWS
 N  OBJ
 G  c1
COLUMNS
    y         c1        2
    y         OBJ       5
    x2        c1        3
    x2        OBJ       6
RHS
    rhs       c1        0
RANGES
BOUNDS
 FX bounds    y         18
 FX bounds    x2        1.3
ENDATA

It seems that, once the parameter goes to the ParameterToEqualToBridge the name information is lost

[odow]

I don't think we have a convention for what to do with bridged names.

[joaquimg]

Do we bridge the variables themselves? or just the constraints?
The bridge does not seem to create variables, so why would the name be lost?

[odow]

It's implemented as a variable bridge: https://github.com/jump-dev/MathOptInterface.jl/blob/master/src/Bridges/Variable/bridges/ParameterToEqualToBridge.jl

[odow]

So the issue is here:

MathOptInterface.jl/src/Bridges/bridge_optimizer.jl

Lines 1687 to 1700 in bc869ec

	function MOI.set(
	b::AbstractBridgeOptimizer,
	attr::MOI.VariableName,
	vi::MOI.VariableIndex,
	name::String,
	)
	if is_bridged(b, vi)
	b.var_to_name[vi] = name
	b.name_to_var = nothing # Invalidate the name map.
	else
	MOI.set(b.model, attr, vi, name)
	end
	return
	end

If the variable is bridged we don't attempt to set the variable name of the inner.

I don't know that I'm strongly motivated to fix this.