public async Task<IHttpActionResult> PutCircle(int id, Circle circle)
        {
            if (!ModelState.IsValid)
            {
                return BadRequest(ModelState);
            }

            if (id != circle.Id)
            {
                return BadRequest();
            }

            db.Entry(circle).State = EntityState.Modified;

            try
            {
                await db.SaveChangesAsync();
            }
            catch (DbUpdateConcurrencyException)
            {
                if (!CircleExists(id))
                {
                    return NotFound();
                }
                else
                {
                    throw;
                }
            }

            return StatusCode(HttpStatusCode.NoContent);
        }
        public async Task<IHttpActionResult> PostCircle(Circle circle)
        {
            if (!ModelState.IsValid)
            {
                return BadRequest(ModelState);
            }

            db.Circles.Add(circle);
            await db.SaveChangesAsync();

            return CreatedAtRoute("DefaultApi", new { id = circle.Id }, circle);
        }