Add warning and improve docs for modify-then-query

docs/src/manual/solutions.md

@@ -286,30 +286,79 @@ Solution is optimal
 
 ## OptimizeNotCalled errors
 
-Modifying a model after calling [`optimize!`](@ref) will reset the model into
-the `MOI.OPTIMIZE_NOT_CALLED` state. If you attempt to query solution
+Due to differences in how solvers cache solutions internally, modifying a model
+after calling [`optimize!`](@ref) will reset the model into the
+`MOI.OPTIMIZE_NOT_CALLED` state. If you then attempt to query solution
 information, an `OptimizeNotCalled` error will be thrown.
 
 If you are iteratively querying solution information and modifying a model,
 query all the results first, then modify the problem.
 
 For example, instead of:
-```julia
-model = Model(GLPK.Optimizer)
-@variable(model, x >= 0)
-optimize!(model)
-set_lower_bound(x, 1)  # This will modify the model
-x_val = value(x)       # This will fail because the model has been modified
-set_start_value(x, x_val)
+```jldoctest
+julia> model = Model(GLPK.Optimizer);
+
+julia> @variable(model, x >= 0);
+
+julia> optimize!(model);
+
+julia> termination_status(model)
+OPTIMAL::TerminationStatusCode = 1
+
+julia> set_upper_bound(x, 1)
+
+julia> x_val = value(x)
+ERROR: OptimizeNotCalled()
+Stacktrace:
+[...]
+
+julia> termination_status(model)
+OPTIMIZE_NOT_CALLED::TerminationStatusCode = 0
 ```
 do
-```julia
-model = Model(GLPK.Optimizer)
-@variable(model, x >= 0)
-optimize!(model)
-x_val = value(x)
-set_lower_bound(x, 1)
-set_start_value(x, x_val)
+```jldoctest
+julia> model = Model(GLPK.Optimizer);
+
+julia> @variable(model, x >= 0);
+
+julia> optimize!(model);
+
+julia> x_val = value(x)
+0.0
+
+julia> termination_status(model)
+OPTIMAL::TerminationStatusCode = 1
+
+julia> set_upper_bound(x, 1)
+
+julia> set_lower_bound(x, x_val)
+
+julia> termination_status(model)
+OPTIMIZE_NOT_CALLED::TerminationStatusCode = 0
+```
+
+If you know that your particular solver supports querying solution information
+after modifications, you can use [`direct_model`](@ref) to bypass the
+`MOI.OPTIMIZE_NOT_CALLED` state:
+```jldoctest
+julia> model = direct_model(GLPK.Optimizer());
+
+julia> @variable(model, x >= 0);
+
+julia> optimize!(model)
+
+julia> termination_status(model)
+OPTIMAL::TerminationStatusCode = 1
+
+julia> set_upper_bound(x, 1)
+
+julia> x_val = value(x)
+0.0
+
+julia> set_lower_bound(x, x_val)
+
+julia> termination_status(model)
+OPTIMAL::TerminationStatusCode = 1
 ```
 
 ```@meta

src/JuMP.jl

@@ -1234,22 +1234,30 @@ Return the value of the attribute `attr` from the model's MOI backend.
 function MOI.get(model::Model, attr::MOI.AbstractModelAttribute)
     if !MOI.is_set_by_optimize(attr)
         return MOI.get(backend(model), attr)
-    elseif attr isa MOI.TerminationStatus
-        if model.is_model_dirty && mode(model) != DIRECT
-            return MOI.OPTIMIZE_NOT_CALLED
-        end
-        return MOI.get(backend(model), attr)
-    elseif attr isa MOI.PrimalStatus || attr isa MOI.DualStatus
-        if model.is_model_dirty && mode(model) != DIRECT
-            return MOI.NO_SOLUTION
-        end
-        return MOI.get(backend(model), attr)
-    else
-        if model.is_model_dirty && mode(model) != DIRECT
-            throw(OptimizeNotCalled())
-        end
-        return _moi_get_result(backend(model), attr)
+    elseif model.is_model_dirty && mode(model) != DIRECT
+        @warn(
+            "The model has been modified since the last call to `optimize!` (" *
+            "or `optimize!` has not been called yet). If you are iteratively " *
+            "querying solution information and modifying a model, query all " *
+            "the results first, then modify the model.",
+        )
+        throw(OptimizeNotCalled())
     end
+    return _moi_get_result(backend(model), attr)
+end
+
+function MOI.get(model::Model, attr::MOI.TerminationStatus)
+    if model.is_model_dirty && mode(model) != DIRECT
+        return MOI.OPTIMIZE_NOT_CALLED
+    end
+    return MOI.get(backend(model), attr)
+end
+
+function MOI.get(model::Model, attr::Union{MOI.PrimalStatus,MOI.DualStatus})
+    if model.is_model_dirty && mode(model) != DIRECT
+        return MOI.NO_SOLUTION
+    end
+    return MOI.get(backend(model), attr)
 end
 
 """