The reason you are getting these errors is that the parent json object is not expecting a result set as one of its inputs, you need to have simple object pairs like {name, string} etc bug report – may be available in future functionality… this just means that you need to convert your multi row results into a concatination of results separated by commas and then converted into a json array.
You almost had it with your second example.
You can achieve what you are after with the GROUP_CONCAT function
select json_object(
'id',p.id
,'desc',p.`desc`
,'child_objects',json_array(
(select GROUP_CONCAT(
json_object('id',id,'parent_id',parent_id,'desc',`desc`)
)
from child_table
where parent_id = p.id))
)
from parent_table p;
This almost works, it ends up treating the subquery as a string which leaves the escape characters in there.
'{\"id\": 1,
\"desc\": \"parent row 1\",
\"child_objects\":
[\"
{\\\"id\\\": 1,
\\\"desc\\\": \\\"child row 1\\\",
\\\"parent_id\\\": 1
},
{\\\"id\\\": 2,
\\\"desc\\\": \\\"child row 2\\\",
\\\"parent_id\\\": 1}\"
]
}'
In order to get this working in an appropriate format, you need to change the way you create the JSON output as follows:
select json_object(
'id',p.id
,'desc',p.`desc`
,'child_objects',(select CAST(CONCAT('[',
GROUP_CONCAT(
JSON_OBJECT(
'id',id,'parent_id',parent_id,'desc',`desc`)),
']')
AS JSON) from child_table where parent_id = p.id)
) from parent_table p;
This will give you the exact result you require:
'{\"id\": 1,
\"desc\": \"parent row 1\",
\"child_objects\":
[{\"id\": 1,
\"desc\": \"child row 1\",
\"parent_id\": 1
},
{\"id\": 2,
\"desc\": \"child row 2\",
\"parent_id\": 1
}]
}'