Prioritizing Constraints in linear SolverContext Model using SolverFoundation - ms-solver-foundation

Question

Good morning,
I am trying to solve what I believe is a linear problem, using Microsofts SolverFoundation in code (c#). I see most references to this type of solving are related to the Solver inside of Excel, and it indeed does share many similarities. However, I have written up an example of what I am trying to do.
For this example, lets say I have 3 grocery stores (Albertsons, Safeway, Costco), and 3 different types of apples (Red, Green, Fuji).
public enum AppleType
{
Red,
Green,
Fuji,
}
Each of these stores offers these different apple types at different percentages per order.
public class Store
{
public string Name { get; set; }
public List<Apple> ApplesOffered { get; set; }
}
public class Apple
{
public AppleType AppleType { get; set; }
public double OrderPercent { get; set; }
}
Here is my mock-data for this setup.
List<Store> stores = new List<Store>
{
new Store()
{
Name = "Albertsons",
ApplesOffered = new List<Apple>
{
new Apple(){AppleType = AppleType.Red, OrderPercent = 80 },
new Apple(){AppleType = AppleType.Green, OrderPercent = 15 },
new Apple(){AppleType = AppleType.Fuji, OrderPercent = 0 }
}
},
new Store()
{
Name = "Safeway",
ApplesOffered = new List<Apple>
{
new Apple(){AppleType = AppleType.Red, OrderPercent = 12 },
new Apple(){AppleType = AppleType.Green, OrderPercent = 30 },
new Apple(){AppleType = AppleType.Fuji, OrderPercent = 0 }
}
},
new Store()
{
Name = "Costco",
ApplesOffered = new List<Apple>
{
new Apple(){AppleType = AppleType.Red, OrderPercent = 10 },
new Apple(){AppleType = AppleType.Green, OrderPercent = 35 },
new Apple(){AppleType = AppleType.Fuji, OrderPercent = 40 }
}
}
};
Alright, so say I have a list of apple types and I want to choose how many of each I want, and the solver should give me the minimal optimized orders from different stores to get what I want.
my code for building the constraints is as follows:
var context = SolverContext.GetContext();
context.ClearModel();
var model = context.CreateModel();
// Decisions
stores.ForEach(store => model.AddDecisions(new Decision(Domain.RealNonnegative, store.Name)));
// Constraints
var constraints = new List<CustomAppleConstraint>();
stores.ForEach(store =>
{
foreach (AppleType a in (AppleType[])Enum.GetValues(typeof(AppleType)))
{
var ao = store.ApplesOffered.FirstOrDefault(_ => _.AppleType == a);
if (ao != null && ao.OrderPercent > 0)
{
constraints.Add(new CustomAppleConstraint
{
Type = a,
Value = $"0.{ao.OrderPercent} * {store.Name}"
});
}
}
});
// Add Constraints to model
var constraintGroups = constraints.GroupBy(_ => _.Type).ToList();
foreach (AppleType a in (AppleType[])Enum.GetValues(typeof(AppleType)))
{
var group = constraintGroups.FirstOrDefault(_ => _.Key == a);
if (group != null)
{
model.AddConstraint($"_{a}", $"{(string.Join(" + ", group.Select(_ => _.Value).ToArray()))} >= {order[a]}");
}
}
// Solve
var solution = context.Solve(new SimplexDirective());
var solutionResults = new List<KeyValuePair<string, double>>();
foreach (var decision in solution.Decisions)
{
var value = (double)decision.GetValues().First()[0];
solutionResults.Add(new KeyValuePair<string, double>(decision.Name, value));
}
return solutionResults;
I have tested this with a bunch of different orders, and it all appears to be giving me the correct data.
Now say I have the following simple order where I only want green apples:
{AppleType.Red, 0},
{AppleType.Green, 10},
{AppleType.Fuji, 0}
I get back a result suggesting 28.57 orders from Costco, which I would expect because Costco offers the highest percentage of green apples per order.
So here is where I am trying to figure the correct way to implement one more constraint.
Say I have some preferences for which stores I use to get certain apple types. (eg, I only want my green apples from Safeway).
// Apple Type : Preferred Store
var orderPrefs = new Dictionary<AppleType, string>()
{
{AppleType.Red, "Albertsons"},
{AppleType.Green, "Safeway" },
{AppleType.Fuji, "Costco" }
};
So even though Costco provides the highest percentage of green apples, I want to add some constraints from my preferences that prioritize the green apple result from Safeway. In this instance, my prefs say I Only want green apple from Safeway, so 100% of this order should come from Safeway.
Granted this is a very simple example, but its the meat of what I am trying to wrap my head around.
I hope this makes sense... I have been trying to figure this out for a few days now with no luck. Thank you.