EF6: Single relationship table for multiple related entities - entity-framework

Question

I have a EF Model with many entities, like Nodes, Attributes, Tags, etc.
There is also an "Alias" entity, and pretty much every other entity else can have a many-to-many relationship with Aliases. One of the undesired things about this is the number of tables that are created to track these relationships (eg. NodeAlias, AttributeAlias, etc.).
Are there any design alternatives that could map an Alias to all of the other entities in a single table? I was thinking maybe something along these lines if it's possible:
+---------+--------+-------------+-----------+
| AliasId | NodeId | AttributeId | TagId |
+---------+--------+-------------+-----------+
| 1 | 1 | 2 | 3 |
+---------+--------+-------------+-----------+

Answer 1

I updated my solution to provide many-to-many relationships between aliases and every other entity.
I intentionally posted this as a separate answer so that my previous answer can also remain here if anyone would need it.
Step #1: I created extension methods for getting and setting property values using reflection in a convenient way:
public static class ObjectExtensions
{
public static TResult GetPropertyValue<TResult>(this object entity, string propertyName)
{
object propertyValue = entity?.GetType().GetProperty(propertyName)?.GetValue(entity);
try
{
return (TResult)propertyValue;
}
catch
{
return default(TResult);
}
}
public static void SetPropertyValue(this object entity, string propertyName, object value)
{
entity?.GetType().GetProperty(propertyName)?.SetValue(entity, value);
}
}
Step #2: I updated the models to provide many-to-many relationship.
public class Node
{
[Key]
public int NodeId { get; set; }
public string Name { get; set; }
public virtual ICollection<AliasMapping> AliasMappings { get; set; }
}
public class Attribute
{
[Key]
public int AttributeId { get; set; }
public string Name { get; set; }
public virtual ICollection<AliasMapping> AliasMappings { get; set; }
}
public class Tag
{
[Key]
public int TagId { get; set; }
public string Name { get; set; }
public virtual ICollection<AliasMapping> AliasMappings { get; set; }
}
public class Alias
{
[Key]
public int AliasId { get; set; }
public string Name { get; set; }
public virtual ICollection<AliasMapping> AliasMappings { get; set; }
}
public class AliasMapping
{
[Key]
public int Id { get; set; }
[ForeignKey("Alias")]
public int AliasId { get; set; }
public Alias Alias { get; set; }
[ForeignKey("Node")]
public int? NodeId { get; set; }
public virtual Node Node { get; set; }
[ForeignKey("Attribute")]
public int? AttributeId { get; set; }
public virtual Attribute Attribute { get; set; }
[ForeignKey("Tag")]
public int? TagId { get; set; }
public virtual Tag Tag { get; set; }
}
Step #3: Due to relationship changes the MyDbContext could have been simplified as the [ForeignKey] data annotations are enough.
public class MyDbContext : DbContext
{
public DbSet<Node> Nodes { get; set; }
public DbSet<Attribute> Attributes { get; set; }
public DbSet<Tag> Tags { get; set; }
public DbSet<Alias> Aliases { get; set; }
public DbSet<AliasMapping> AliasMappings { get; set; }
}
Step #4: I also updated the extension methods so that you can create and remove alias mappings.
public static class AliasExtensions
{
public static void CreateMapping(this MyDbContext context, object entity, Alias alias)
{
if (entity == null || alias == null)
{
return;
}
string mappingEntityPropertyName = entity.GetType().Name;
string entityKeyPropertyName = String.Concat(mappingEntityPropertyName, "Id");
int entityId = entity.GetPropertyValue<int>(entityKeyPropertyName);
AliasMapping[] mappings =
context
.AliasMappings
.Where(mapping => mapping.AliasId == alias.AliasId)
.ToArray();
if (mappings.Any(mapping => mapping.GetPropertyValue<int?>(entityKeyPropertyName) == entityId))
{
// We already have the mapping between the specified entity and alias.
return;
}
bool usableMappingExists = true;
var usableMapping = mappings.FirstOrDefault(mapping => mapping.GetPropertyValue<int?>(entityKeyPropertyName) == null);
if (usableMapping == null)
{
usableMappingExists = false;
usableMapping = new AliasMapping()
{
Alias = alias
};
}
usableMapping.SetPropertyValue(mappingEntityPropertyName, entity);
usableMapping.SetPropertyValue(entityKeyPropertyName, entityId);
if (!usableMappingExists)
{
context.AliasMappings.Add(usableMapping);
}
// This step is required here, I think due to using reflection.
context.SaveChanges();
}
public static void RemoveMapping(this MyDbContext context, object entity, Alias alias)
{
if (entity == null || alias == null)
{
return;
}
string mappingEntityPropertyName = entity.GetType().Name;
string entityKeyPropertyName = String.Concat(mappingEntityPropertyName, "Id");
int entityId = entity.GetPropertyValue<int>(entityKeyPropertyName);
AliasMapping[] mappings =
context
.AliasMappings
.Where(mapping => mapping.AliasId == alias.AliasId)
.ToArray();
AliasMapping currentMapping = mappings.FirstOrDefault(mapping => mapping.GetPropertyValue<int?>(entityKeyPropertyName) == entityId);
if (currentMapping == null)
{
// There is no mapping between the specified entity and alias.
return;
}
currentMapping.SetPropertyValue(mappingEntityPropertyName, null);
currentMapping.SetPropertyValue(entityKeyPropertyName, null);
// This step is required here, I think due to using reflection.
context.SaveChanges();
}
}
Step #5: Updated the console app steps to align it with the changes.
class Program
{
static void Main(string[] args)
{
// Consider specify the appropriate database initializer!
// I use DropCreateDatabaseAlways<> strategy only for this example.
Database.SetInitializer(new DropCreateDatabaseAlways<MyDbContext>());
var aliases =
Enumerable
.Range(1, 9)
.Select(index => new Alias() { Name = String.Format("Alias{0:00}", index) })
.ToList();
var attributes =
Enumerable
.Range(1, 5)
.Select(index => new Attribute() { Name = String.Format("Attribute{0:00}", index) })
.ToList();
var nodes =
Enumerable
.Range(1, 5)
.Select(index => new Node() { Name = String.Format("Node{0:00}", index) })
.ToList();
var tags =
Enumerable
.Range(1, 5)
.Select(index => new Tag() { Name = String.Format("Tag{0:00}", index) })
.ToList();
using (var context = new MyDbContext())
{
context.Aliases.AddRange(aliases);
context.Nodes.AddRange(nodes);
context.Attributes.AddRange(attributes);
context.Tags.AddRange(tags);
// Always save changes after adding an entity but before trying to create a mapping.
context.SaveChanges();
// One Alias To Many Entities
context.CreateMapping(nodes[0], aliases[0]);
context.CreateMapping(nodes[1], aliases[0]);
context.CreateMapping(nodes[2], aliases[0]);
context.CreateMapping(nodes[3], aliases[0]);
context.CreateMapping(attributes[0], aliases[0]);
context.CreateMapping(attributes[1], aliases[0]);
context.CreateMapping(attributes[2], aliases[0]);
context.CreateMapping(tags[0], aliases[0]);
context.CreateMapping(tags[1], aliases[0]);
// One Entity To Many Aliases
context.CreateMapping(nodes[4], aliases[0]);
context.CreateMapping(nodes[4], aliases[1]);
context.CreateMapping(nodes[4], aliases[2]);
context.CreateMapping(attributes[3], aliases[1]);
context.CreateMapping(attributes[3], aliases[3]);
context.CreateMapping(tags[2], aliases[2]);
context.CreateMapping(tags[2], aliases[3]);
// Remove mapping
context.RemoveMapping(nodes[4], aliases[0]);
// Not really needed here as both 'CreateMapping' and 'RemoveMapping' save the changes
context.SaveChanges();
}
Console.Write("Press any key to continue . . .");
Console.ReadKey(true);
}
}
Please note: RemoveMapping() will not delete an AliasMapping even if no entity is associated with it! But CreateMapping() will make use of it later if needed. E.g. look at the screenshot below and check AliasMapping where Id = 5.
Screenshot about the execution result:

Answer 2

You were talking about many-to-many relationship but reading your post I think it is more likely a "special one-to-many" relationship, actually "combined multiple one-to-one" relationship as I see that an Alias can be mapped to a single Node AND/OR to a single Attribute AND/OR to a single Tag.
I think I found a solution for this case.
If it's not the case and an Alias can be mapped to multiple Node AND/OR to multiple Attribute AND/OR to multiple Tag then I think this solution below needs only a small change. :)
Step #1 - These are my example models
public class Node
{
[Key]
public int Id { get; set; }
public string Name { get; set; }
public virtual AliasMapping AliasMapping { get; set; }
}
public class Attribute
{
[Key]
public int Id { get; set; }
public string Name { get; set; }
public virtual AliasMapping AliasMapping { get; set; }
}
public class Tag
{
[Key]
public int Id { get; set; }
public string Name { get; set; }
public virtual AliasMapping AliasMapping { get; set; }
}
public class Alias
{
[Key]
public int AliasId { get; set; }
public string Name { get; set; }
public virtual AliasMapping AliasMapping { get; set; }
}
Step #2 - Creating the custom mapping table
public class AliasMapping
{
[Key]
[ForeignKey("Alias")]
public int AliasId { get; set; }
public Alias Alias { get; set; }
[ForeignKey("Node")]
public int NodeId { get; set; }
public virtual Node Node { get; set; }
[ForeignKey("Attribute")]
public int AttributeId { get; set; }
public virtual Attribute Attribute { get; set; }
[ForeignKey("Tag")]
public int TagId { get; set; }
public virtual Tag Tag { get; set; }
}
Step #3 - Creating the DbContext
public class MyDbContext : DbContext
{
public DbSet<Node> Nodes { get; set; }
public DbSet<Attribute> Attributes { get; set; }
public DbSet<Tag> Tags { get; set; }
public DbSet<Alias> Aliases { get; set; }
public DbSet<AliasMapping> AliasMappings { get; set; }
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder
.Entity<AliasMapping>()
.HasOptional(mapping => mapping.Attribute)
.WithOptionalPrincipal(attribute => attribute.AliasMapping)
.Map(config => config.MapKey("AliasId"));
modelBuilder
.Entity<AliasMapping>()
.HasOptional(mapping => mapping.Node)
.WithOptionalPrincipal(node => node.AliasMapping)
.Map(config => config.MapKey("AliasId"));
modelBuilder
.Entity<AliasMapping>()
.HasOptional(mapping => mapping.Tag)
.WithOptionalPrincipal(tag => tag.AliasMapping)
.Map(config => config.MapKey("AliasId"));
}
}
Step #4 - Creating extension method so that creating a relationship will be easy
public static class AliasExtensions
{
public static void CreateMapping<TEntity>(this MyDbContext context, TEntity entity, Alias alias)
{
string mappingEntityPropertyName = typeof(TEntity).Name;
string entityKeyPropertyName = String.Concat(mappingEntityPropertyName, "Id");
bool entityExists = true;
var mapping = context.AliasMappings.Find(alias.AliasId);
if (mapping == null)
{
entityExists = false;
mapping = new AliasMapping()
{
Alias = alias
};
}
typeof(AliasMapping)
.GetProperty(mappingEntityPropertyName)
.SetValue(mapping, entity);
typeof(AliasMapping)
.GetProperty(entityKeyPropertyName)
.SetValue(mapping, typeof(TEntity).GetProperty("Id").GetValue(entity));
if (!entityExists)
{
context.AliasMappings.Add(mapping);
}
}
}
Step #5 - Created a console app to see this working
class Program
{
static readonly Random rnd = new Random(DateTime.Now.TimeOfDay.Milliseconds);
static void Main(string[] args)
{
Database.SetInitializer(new DropCreateDatabaseAlways<MyDbContext>());
var aliases =
Enumerable
.Range(1, 9)
.Select(index => new Alias() { Name = String.Format("Alias{0:00}", index) })
.ToList();
var attributes =
Enumerable
.Range(1, 5)
.Select(index => new Attribute() { Name = String.Format("Attribute{0:00}", index) })
.ToList();
var nodes =
Enumerable
.Range(1, 5)
.Select(index => new Node() { Name = String.Format("Node{0:00}", index) })
.ToList();
var tags =
Enumerable
.Range(1, 5)
.Select(index => new Tag() { Name = String.Format("Tag{0:00}", index) })
.ToList();
using (var context = new MyDbContext())
{
context.Aliases.AddRange(aliases);
context.Nodes.AddRange(nodes);
context.Attributes.AddRange(attributes);
context.Tags.AddRange(tags);
context.SaveChanges();
// Associate aliases to attributes
attributes.ForEach(attribute =>
{
var usableAliases = aliases.Where(alias => alias.AliasMapping?.Attribute == null).ToList();
var selectedAlias = usableAliases[rnd.Next(usableAliases.Count)];
context.CreateMapping(attribute, selectedAlias);
});
// Associate aliases to nodes
nodes.ForEach(node =>
{
var usableAliases = aliases.Where(alias => alias.AliasMapping?.Node == null).ToList();
var selectedAlias = usableAliases[rnd.Next(usableAliases.Count)];
context.CreateMapping(node, selectedAlias);
});
// Associate aliases to tags
tags.ForEach(tag =>
{
var usableAliases = aliases.Where(alias => alias.AliasMapping?.Tag == null).ToList();
var selectedAlias = usableAliases[rnd.Next(usableAliases.Count)];
context.CreateMapping(tag, selectedAlias);
});
context.SaveChanges();
}
Console.Write("Press any key to continue . . .");
Console.ReadKey(true);
}
}