Multiplicity constraint violation debug

I'm getting a multiplicity constraint violation in my entity model.

On my entity model I have two relationship properties:

1. SubstanceTypeMixConstituents
2. Category

SubstanceTypeMixConstituents

- Multiplicity * (Many)

Category

- Multiplicity: 1 (One)

- Foreign key, not null

How do I find what causing the problem and resolve this issue?

System.InvalidOperationException: A relationship multiplicity constraint violation occurred: An EntityReference expected at least one related object, but the query returned no related objects from the data store.
   at System.Data.Objects.DataClasses.EntityReference`1.Load(MergeOption mergeOption)
   at System.Data.Objects.DataClasses.RelatedEnd.DeferredLoad()
   at System.Data.Objects.Internal.LazyLoadBehavior.LoadProperty[TI开发者_StackOverflow中文版tem](TItem propertyValue, String relationshipName, String targetRoleName, Boolean mustBeNull, Object wrapperObject)
   at System.Data.Objects.Internal.LazyLoadBehavior.<>c__DisplayClass7`2.<GetInterceptorDelegate>b__2(TProxy proxy, TItem item)
   at System.Data.Entity.DynamicProxies.SubstanceType_BEE32ACA75386E981F7CA3F6A3C565BC1D8ADACA228C603A2EACC918DCDCBA30.get_Category()

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As far as I understand, you have two entities - Category and SubstanceTypeMixConstituent which have a One-to-many relationship - a Category can have multiple SubstanceTypeMixConstituents, but a SubstanceTypeMixConstituent can have (and must have) only one Category (correct me if I am wrong). The error message you get means exactly what it says - you are either trying to save a Category with an empty collection of SubstanceTypeMixConstituents, or a SubstanceTypeMixConstituent without a Category.