Simplify write_to_file so that it does not need an optimizer (#591)

Author: odow

src/problems.jl

@@ -306,26 +306,17 @@ end
 
 Write the current problem to the file at `filename`.
 
-Requires solving the problem at least once using [`solve!`](@ref) to ensure that
-the problem is loaded into a MathOptInterface model.
-
 The file format is inferred from the filename extension. Supported file
 types depend on the model type.
 
 Currently, `Float64` is the only supported coefficient type. This may be
 relaxed in future if file formats support other types.
 """
 function write_to_file(p::Problem{T}, filename::String) where {T<:Float64}
-    if p.model === nothing
-        msg = """
-        Problem has not been loaded into a MathOptInterface model;
-        call `solve!(problem, optimizer)` before writing problem to file.
-        """
-        throw(ArgumentError(msg))
-    end
+    src = Context(p, MOI.Utilities.Model{T})
     dest = MOI.FileFormats.Model(; filename)
     model = MOI.Bridges.full_bridge_optimizer(dest, T)
-    MOI.copy_to(model, p.model)
+    MOI.copy_to(model, src.model)
     MOI.write_to_file(dest, filename)
     return
 end

test/test_utilities.jl

@@ -922,8 +922,6 @@ function test_write_to_file()
     p = minimize(logsumexp(x))
     dir = mktempdir()
     filename = joinpath(dir, "test.mof.json")
-    @test_throws ArgumentError write_to_file(p, filename)
-    solve!(p, SCS.Optimizer; silent_solver = true)
     write_to_file(p, filename)
     @test occursin("ExponentialCone", read(filename, String))
     p_int = minimize(logsumexp(x); numeric_type = Int)