Many to Many relationship Entity Framework Core 5 - entity-framework

Question

I created a Blazor project and I have a many-to-many relationship between these classes:
public class ItemAttribute
{
[Key]
public int ItemAttributeId { get; set; }
public string Title { get; set; }
public ICollection<Item> Items { get; set; }
public ICollection<ItemAttributeCluster> itemAttributeClusters { get; set; }
}
and
public class ItemAttributeCluster
{
[Key]
public int ItemAttributeClusterId { get; set; }
public string Titel { get; set; }
public bool IsMultiChoice { get; set; }
public ICollection<ItemAttribute> itemAttributes { get; set; }
}
So far so good, EF generates the Join table ItemAttributeItemAttributeCluster, ok.
Then I try to add a new cluster of ItemAttributes for the first time with my controller:
// Create
[HttpPost]
public async Task<IActionResult> Post(ItemAttributeCluster itemAttributeCluster)
{
_context.ItemAttributeClusters.Add(itemAttributeCluster);
await _context.SaveChangesAsync();
return Ok(itemAttributeCluster);
}
and I get this error:
Cannot insert explicit value for identity column in table 'ItemAttributes' when IDENTITY_INSERT is set to OFF.
What am I doing wrong? Why is EF trying to write something into 'ItemAttributes'? When i´m trying to create a new Cluster on 'ItemAttributesCluster' and the Join Table?
Migration Builder:
Join Table
migrationBuilder.CreateTable(
name: "ItemAttributeItemAttributeCluster",
columns: table => new
{
itemAttributeClustersItemAttributeClusterId = table.Column<int>(type: "int", nullable: false),
itemAttributesItemAttributeId = table.Column<int>(type: "int", nullable: false)
},
constraints: table =>
{
table.PrimaryKey("PK_ItemAttributeItemAttributeCluster", x => new { x.itemAttributeClustersItemAttributeClusterId, x.itemAttributesItemAttributeId });
table.ForeignKey(
name: "FK_ItemAttributeItemAttributeCluster_ItemAttributeClusters_itemAttributeClustersItemAttributeClusterId",
column: x => x.itemAttributeClustersItemAttributeClusterId,
principalTable: "ItemAttributeClusters",
principalColumn: "ItemAttributeClusterId",
onDelete: ReferentialAction.Cascade);
table.ForeignKey(
name: "FK_ItemAttributeItemAttributeCluster_ItemAttributes_itemAttributesItemAttributeId",
column: x => x.itemAttributesItemAttributeId,
principalTable: "ItemAttributes",
principalColumn: "ItemAttributeId",
onDelete: ReferentialAction.Cascade);
});
ItemAttributes
migrationBuilder.CreateTable(
name: "ItemAttributes",
columns: table => new
{
ItemAttributeId = table.Column<int>(type: "int", nullable: false)
.Annotation("SqlServer:Identity", "1, 1"),
Title = table.Column<string>(type: "nvarchar(max)", nullable: true)
},
constraints: table =>
{
table.PrimaryKey("PK_ItemAttributes", x => x.ItemAttributeId);
});
ItemAttributeCluster
migrationBuilder.CreateTable(
name: "ItemAttributeClusters",
columns: table => new
{
ItemAttributeClusterId = table.Column<int>(type: "int", nullable: false)
.Annotation("SqlServer:Identity", "1, 1"),
Titel = table.Column<string>(type: "nvarchar(max)", nullable: true),
IsMultiChoice = table.Column<bool>(type: "bit", nullable: false)
},
constraints: table =>
{
table.PrimaryKey("PK_ItemAttributeClusters", x => x.ItemAttributeClusterId);
});

Answer 1

If this was an existing schema for the ItemAttribute / Cluster tables and their PK were defined as identity columns, you will need to tell EF to expect them using the [Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)] attribute alongside the Key designation.
When using a naming convention that EF recognizes like "ItemAttributeId" or "Id" I believe EF will default to assuming these are Identity columns, but with a name like "ItemAttributeCode" I believe it would assume a database generated option of "None" as default.

Answer 2

try to add some navigation properties
public ItemAttributeCluster()
{
AttributeClusters = new HashSet<AttributeCluster>();
}
[Key]
public int Id { get; set; }
public string Titel { get; set; }
public bool IsMultiChoice { get; set; }
[InverseProperty(nameof(AttributeCluster.ItemAttributeClaster))]
public virtual ICollection<AttributeCluster> AttributeClusters { get; set; }
}
public partial class ItemAttribute
{
public ItemAttribute()
{
AttributeClusters = new HashSet<AttributeCluster>();
}
[Key]
public int Id { get; set; }
public string Title { get; set; }
public virtual ICollection<Item> Items { get; set; }
[InverseProperty(nameof(AttributeCluster.ItemAttribute))]
public virtual ICollection<AttributeCluster> AttributeClusters { get; set; }
}
public partial class AttributeCluster
{
[Key]
public int Id { get; set; }
public int ItemAttributeId { get; set; }
public int ItemAttributeClasterId { get; set; }
[ForeignKey(nameof(ItemAttributeId))]
[InverseProperty("AttributeClusters")]
public virtual ItemAttribute ItemAttribute { get; set; }
[ForeignKey(nameof(ItemAttributeClasterId))]
[InverseProperty(nameof(ItemAttributeCluster.AttributeClusters))]
public virtual ItemAttributeCluster ItemAttributeClaster { get; set;
}
dbcontext (no any fluent apis at all)
public virtual DbSet<AttributeCluster> AttributeClusters { get; set; }
public virtual DbSet<ItemAttribute> ItemAttributes { get; set; }
public virtual DbSet<ItemAttributeCluster> ItemAttributeClusters { get; set; }
Test
var itemAttributeClaster = new ItemAttributeCluster { Titel="titleClaster2", IsMultiChoice=false};
var itemAttribute = new ItemAttribute{Title="attrTitle" };
var attributeClaster = new AttributeCluster { ItemAttribute = itemAttribute, ItemAttributeClaster = itemAttributeClaster };
_context.AttributeClusters.Add(attributeClaster);
_context.SaveChanges();
it created 1 record in each of 3 tables

Answer 3

I give up on getting this to work with ef. I run several sql`s directly to achieve the same functionality and so far it works, not a satisfactory solution but it needs to be done.