I have a JobOffer class which has a many-to-many relationship with the JobOfferTags class.

I am getting the tags as a collection of strings from the client and then creating a collection with their database counterparts.

Collection<JobOfferTag> tags = [];

for (int i = 0; i < offer.Tags.Count; i++)
{
    // Check if tag exists in db

    string tag = offer.Tags.ElementAt(0);
    JobOfferTag? dbTag = await dbContext.JobOfferTags.FirstOrDefaultAsync(t => t.Value == tag);

    if (dbTag == null)
    {
        // ...
    }

    tags.Add(dbTag);
}

public class JobOffer : BaseJobOffer
{
    public int Id { get; set; }
    public DateTimeOffset Created { get; private set; } = DateTimeOffset.UtcNow;
    public new ICollection<JobOfferTag> Tags = [];

    public JobOffer() : base()
    {
    }

    [SetsRequiredMembers]
    public JobOffer(BaseJobOffer offer) : base(offer)
    {
    }
}

Although when I try setting the Tags property to tags, only the first item of tags gets saved when calling DbContext.SaveChangesAsync().

JobOffer newOffer = new(offer)
{
    Tags = tags,
    UserId = user.Id
};

dbContext.JobOffers.Add(newOffer);

await dbContext.SaveChangesAsync();

EF Core only creates a row in the join table for the first tag.