Using two JSON sources for Jekyll output - json

Question

Suppose I have a list of projects (let's call that proj.json) and a list of smaller sub-projects (sub_proj.json) that are associated with the list of projects. The files are as follows:
proj.json
[{
"title": "Project 1",
"description": "aaa.",
"subproj": ["RefSP1", "RefSP2"]
},
{
"title": "Project 2",
"description": "aaa.",
"subproj": ["RefSP3"]
}]
sub_proj.json
[{
"title": "Sub Project 1",
"description": "aaa.",
"key": "RefSP1"
},
{
"title": "Sub Project 2",
"description": "aaa.",
"key": "RefSP2"
},
{
"title": "Sub Project 3",
"description": "aaa.",
"key": "RefSP3"
}]
But in the output.html, I seem to be missing something. The code is below: (I removed it of formatting for simplicity)
{% for item in site.data.proj %}
{{ item.title }}
{% assign key_array = item.subproj %}
{% for sub in key_array %}
{% assign subproject = site.data.sub_proj | where:"key",sub %}
{{ subproject.title | inspect }}
{% endfor %}
I seem to get the correct number of subprojects using {{ subproject.title | inspect }}, but they're all nil. Adding jsonify doesn't really do anything.
edit:
Printing out {{ item | inspect }}
results in
{ "title"=>"Project 1","description"=>"aaa.","subproj"=>["RefSP1","RefSP2"]}
But {{ subproject | inspect }}
comes out as
[{"title"=>"Sub Project 1","description"=>"aaa.","key"=>"RefSP1"}] [{"title"=>"Sub Project 2","description"=>"aaa.","key"=>"RefSP2"}]
The square brackets are the main difference. I tried removing them through filters, still nothing.