在控制器中，当您循环通过会议参数Meeting时，@meeting不会保存到数据库，并且@meeting变量被覆盖，直到数组中的最后一项...

@meeting.each do |m|     
  @meeting = Meeting.new(member_id: m[:meeting][:member_id], tournament_id: m[:meeting][:tournament_id], attendance: m[:meeting][:attendance])
end

......而那个人被救了。

if @meeting.save

也不知道你的show操作是怎么回事，只是保存后不要重定向到它。
在Rails表单中使用数组有点不确定，但它在这里。
控制器：

# params - submitted params from the form
#          { "authenticity_token"=>"[FILTERED]", 
#             "meetings"=>[
#               { "member_id"=>"1", "tournament_id"=>"1", attendance"=>"1" },
#               ...
#              ],
#            "commit"=>"Save"
#          }

# POST /meetings
def create
  @meetings = meetings_params.map do |param|
    Meeting.create_with(attendance: param[:attendance])
      .find_or_create_by(member_id: param[:member_id], tournament_id: param[:tournament_id])
  end
  respond_to do |format|
    if @meetings.detect{|m| m.id == nil }  # id=nil if any meetings didn't save
      # FIXME: @tournament not initialized here
      format.html { render :new, status: :unprocessable_entity }
    else
      format.html { redirect_to meetings_url, notice: "Meetings were created." }
    end
  end
end

# Returns array 
# [{member_id: 1, tournament_id: 1, attendance: 1}, ...]
def meetings_params
  params.permit(meetings: [:member_id, :tournament_id, :attendance]).require(:meetings)
end

表格：

<%= form_with url: "/meetings" do |f| %>
  <% Member.where(active: true).each do |member| %>
    <%# TODO: if meeting doesn't save, there is no error to show %>
    <%= text_field_tag "meetings[][member_id]",     member.id %>
    <%= text_field_tag "meetings[][tournament_id]", @tournament.id %>
    <%= check_box_tag  "meetings[][attendance]",    Meeting.find_by(member: member, tournament: @tournament)&.attendance %>
  <% end %>
  <%= f.submit %>
<% end %>

或使用fields_for帮助器：

<%= form_with url: "/meetings" do |f| %>
  <% Member.where(active: true).each do |member| %>
    <%#                            this nil does the [] thing %>
    <%#                                     |                 %>
    <%= fields_for "meetings", nil, index: nil do |ff| %>
      <%= ff.number_field "member_id",     value: member.id %>
      <%= ff.hidden_field "tournament_id", value: @tournament.id %>
      <%= ff.check_box    "attendance",    value: Meeting.find_by(member: member, tournament: @tournament)&.attendance %>
    <% end %>
  <% end %>
  <%= f.submit %>
<% end %>

更新

可以说，以上所有这些都是非常不合常理的。下面是我想出的让它变得超级简单的方法：
我们需要一个Attendance表来跟踪每个成员对每个Meeting的出席情况：
一个
MeetingsController是默认支架：

bin/rails g scaffold_controller meeting

仅更新new操作：

# app/controllers/meetings_controller.rb
def new
  @meeting = Meeting.new
  # NOTE: build a list of attendances for the form to render
  Member.where(active: true).each do |member|
    @meeting.attendances.build(member: member)
  end
end

现在的表单非常简单：

<!-- app/views/meetings/_form.html.erb -->
<%= form_with model: @meeting do |f| %>
  <%= f.fields_for :attendances do |ff| %>
    <%= ff.number_field :member_id %>
    <%= ff.check_box :attended %>
  <% end %>
  <%= f.submit %>
<% end %>

create和update完全独立工作。