Entity Framework Many to Many and existing data post from angular - entity-framework

Question

Entity Framework from Database First created the Table model classes having many to many relationships in C# WebApi.
Table ACCOUNTS and table METADATA have a many-to-many relationship between them.
I want to add a new entry on ACCOUNTS table and link this entry with some existing entries from METADATA table. How can I do this using AngularJS to post data?
I am sending this data on $http:
var account: {
Title: 'Title',
User: 'User',
METADATA: [
{
Name: 'value1'
},
{
Name: 'value2'
}]
}
The account variable above is based on ACCOUNTS class which is being read by the C# web api using POST and [FromBody] like this:
[HttpPost]
public async Task<IHttpActionResult> Add([FromBody]ACCOUNTS account)
{
db.ACCOUNTS.Add(account);
await db.SaveChangesAsync();
int accountId = account.AccountId;
return Ok(accountId);
}
I am getting an error of primary key violation of existence of value1 and value2 on table METADATA.
This is correct because the values exist in the table.
But actually I want these values to be linked to the "intermediate table" which links ACCOUNTS and METADATA as many-to-many relationship, and not to be added.
Any solution to this scenario?

Answer 1

Before inserting the passed disconnected account object, you need to map the child METADATA objects to existing database entities. For instance, using something like this:
var medataNames = account.METADATA.Select(e => e.Name);
account.METADATA = db.METADATA.Where(e => metadataNames.Contains(e.Name)).ToList();
db.ACCOUNTS.Add(account);
// ...