Mysql2::Error: Column ‘project_id’ in where clause is ambiguous

把id进行精确：

WHERE project.id = &#39;3&#39;

使用joins进行查询：

&nbsp; @emails = @emails.joins(:project).where(&quot;projects.id = ?&quot;, params[:project_id]) if params[:project_id].present?

视图页面：

&lt;%= form_tag emails_path, method: &#39;GET&#39; do |f| %&gt;
&nbsp; &lt;b&gt;search by project:&lt;/b&gt;&lt;br/&gt;
&nbsp; Project：&lt;%= select_tag :project_id, options_from_collection_for_select(Project.all, :id, :name, params[:project_id]), include_blank: &#39;all&#39;, class: &#39;form_select&#39; %&gt;
&nbsp; &lt;br/&gt;
&nbsp; &lt;%= submit_tag &#39;search&#39; %&gt;
&lt;% end %&gt;