How to check if two entity entries of a collection are the same

Question

I'm trying to do a generic method to update Entity Framework collections, one to many. I did this method, but i'm having a problem, when i try to verify, if the element in the new collection already exists in the old one. If exists, i have to update it, rather then remove and add it again.

The code is this:

public TEntity UpdateCollection<TEntity, TChildren>(myappContext dbContext, TEntity parentObject, Expression<Func<TEntity, 
            ICollection<TChildren>>> propertyExpression, ICollection<TChildren> objectChilren) where TEntity : class where TChildren : class
        {

            var parentEntityObject = dbContext.Entry<TEntity>(parentObject);
            List<TChildren> originalChildrenData = parentEntityObject.Collection(propertyExpression).CurrentValue.ToList();

            // Updating or removing existing items
            foreach (var originalItem in originalChildrenData)
            {
                // Where the problem is: If entry was just modified, i have to update.
                var newItem = objectChilren.FirstOrDefault(x => x == originalItem);
                if (newItem != null)
                {
                    dbContext.Entry<TChildren>(originalItem).CurrentValues.SetValues(newItem);
                    dbContext.Entry<TChildren>(originalItem).State = System.Data.EntityState.Modified;
                }
                else
                {
                    dbContext.Entry<TChildren>(originalItem).State = System.Data.EntityState.Deleted;
                }
            }

            // Adding new items
            foreach(var newItem in objectChilren.Except(originalChildrenData)){
                parentEntityObject.Collection(propertyExpression).CurrentValue.Add(newItem);
            }

            parentEntityObject.State = System.Data.EntityState.Modified;
            return parentEntityObject.Entity;
        }

Instead of try to check with:

var newItem = objectChilren.FirstOrDefault(x => x == originalItem);
                    if (newItem != null)

I also tryed with:

var newItem = this.Set<TChildren>().Local.FirstOrDefault(x => x == originalItem);

But also doesn't work, always returns null. I have to get the corresponding entry and only update it.

If it's not possible, there is another generic way to update collections "one to many"?

Try to compare the primary key property of the object rather than the object itself which I assume does not override Equals and GetHashKey. — Magnus, Jun 06 '15 at 18:20
I thought it too, but how to know which are de keys? and compare them? It needs to be generic, so i can't define a primary key. I have to find which one is the key and after compare — Rodrigo Leite, Jun 06 '15 at 18:23
See this [Entity Framework 4 How to find the primary key](http://stackoverflow.com/questions/2958921/entity-framework-4-how-to-find-the-primary-key) — Magnus, Jun 06 '15 at 18:29
thank you, I did some think like it: http://stackoverflow.com/questions/7253943/entity-framework-code-first-find-primary-key — Rodrigo Leite, Jun 07 '15 at 18:15

score 2 · Accepted Answer · answered Jun 07 '15 at 18:23

I did it, comparing the key property from my collection, as Magnus suggested, like this:

public TEntity UpdateCollection<TEntity, TChildren>(myappContext dbContext, TEntity parentObject, Expression<Func<TEntity, 
            ICollection<TChildren>>> propertyExpression, ICollection<TChildren> objectChilren) where TEntity : class where TChildren : class
        {

            var parentEntityObject = dbContext.Entry<TEntity>(parentObject);
            List<TChildren> originalChildrenData = parentEntityObject.Collection(propertyExpression).CurrentValue.ToList();

            // Get key name
            var entityKeyName = GetKeyName(dbContext, originalChildrenData.Union(objectChilren).First());

            // Updating or removing existing items
            foreach (var originalItem in originalChildrenData)
            {

                var originalValueKey = originalItem.GetType().GetProperty(entityKeyName).GetValue(originalItem, null);
                var itemCompareExpression = GetCompareExpression<TChildren>(entityKeyName, originalValueKey);

                // If entry was just modified, i have to update.
                var newItem = objectChilren.FirstOrDefault(itemCompareExpression.Compile());
                if (newItem != null)
                {
                    dbContext.Entry<TChildren>(originalItem).CurrentValues.SetValues(newItem);
                    dbContext.Entry<TChildren>(originalItem).State = System.Data.EntityState.Modified;
                    // Remove item, because only 'new items' will be added after this loop
                    objectChilren.Remove(newItem);
                }
                else
                {
                    dbContext.Entry<TChildren>(originalItem).State = System.Data.EntityState.Deleted;
                }
            }

            // Adding new items
            foreach(var newItem in objectChilren)
            {
                parentEntityObject.Collection(propertyExpression).CurrentValue.Add(newItem);
            }

            parentEntityObject.State = System.Data.EntityState.Modified;
            return parentEntityObject.Entity;
        }

Methods called:

public string GetKeyName<TEntity>(myappContext dbContext, TEntity entity) where TEntity : class
        {
            ObjectContext objectContext = ((IObjectContextAdapter)dbContext).ObjectContext;
            ObjectSet<TEntity> set = objectContext.CreateObjectSet<TEntity>();
            return set.EntitySet.ElementType.KeyMembers.FirstOrDefault().Name;
        }

        public Expression<Func<TEntity, bool>> GetCompareExpression<TEntity>(string keyName, object value) where TEntity : class
        {
            var parameter = Expression.Parameter(typeof(TEntity), "x");
            var property = Expression.Property(parameter, keyName);
            var method = property.Type.GetMethod("Equals", new[] { property.Type });
            var convertedValue = Convert.ChangeType(value, property.Type);
            var expression = Expression.Call(property, method, Expression.Constant(convertedValue));

            return Expression.Lambda<Func<TEntity, bool>>(expression, parameter);
        }

As all my entities only have one key, i just used "First" on "GetKeyName", but it returns all keys, if necessary.

How to check if two entity entries of a collection are the same

1 Answers1