<form asp-action="SearchByText" asp-route-topicId="@Model.Id">
    <div asp-validation-summary="ModelOnly" class="text-danger"></div>
    <div class="form-group">
        <label name="query" class="col-md-2 control-label"></label>
        <div class="col-md-10">
            <input name="query" class="form-control" />
        </div>
    </div>
    <div class="form-group">
        <input type="submit" value="Search" class="btn btn-default" />
    </div>
</form>

[AllowAnonymous]
        public async Task<IActionResult> SearchByText(Guid? topicId, String query)
        {
            if (topicId == null)
            {
                return NotFound();
            }

            var forumTopic = await context.ForumTopics
                .Include(f => f.Creator)
                .Include(f => f.Forum)
                .Include(f => f.Messages)
                .ThenInclude(f => f.Creator)
                .Include(f => f.Messages)
                .ThenInclude(f => f.Attachments)
                .SingleOrDefaultAsync(m => m.Id == topicId);

            forumTopic.Messages = forumTopic.Messages.Where(p => p.Creator.Email.Equals(query)).ToList();
            if (forumTopic == null)
            {
                return NotFound();
            }

            this.ViewBag.Query = query;
            return View(forumTopic);
        }