Postgres: Convert Object Graphs to JSON - sql

Question

Is there a procedure that converts Postgres Object Graphs to a nested JSON tree?
Context:
An author has many books.
A book has many review.
The query below gets all reviews of all books of all authors:
select author.id, book.title, review.text
from author
left join book on author.id = book.author_id
left join review on book.id = review.book_id;
With my test data set, this returns:
id title text
1 Oathbringer obath
1 Oathbringer oath
1 The Way of Kings a
1 The Way of Kings bye
1 The Way of Kings hi
2 The Eye of the World w
3 The Fellowship of the Ring x
The tabular format is inconvenient, so I convert it to json:
select json_agg(sub2.*) as data from (
select sub.id, sub.title, json_agg(sub.text) as review from (
select author.id, book.title, review.text
from author
left join book on author.id = book.author_id
left join review on book.id = review.book_id
) as sub group by sub.id, sub.title
) as sub2;
The data is fetched as
[
{
"id":1,
"title":"Oathbringer",
"review":[
"obath",
"oath"
]
},
{
"id":1,
"title":"The Way of Kings",
"review":[
"a",
"bye",
"hi"
]
},
{
"id":2,
"title":"The Eye of the World",
"review":[
"w"
]
},
{
"id":3,
"title":"The Fellowship of the Ring",
"review":[
"x"
]
}
]
Unfortunately, the query is not correct because books for an author are not getting aggregated. Attempting to correct this, I wrote this query:
select sub2.id, json_agg((sub2.title, sub2.review)) as data from (
select sub.id, sub.title, json_agg(sub.text) as review from (
select author.id, book.title, review.text
from author
left join book on author.id = book.author_id
left join review on book.id = review.book_id
) as sub group by sub.id, sub.title
) as sub2 group by sub2.id;
But the json keys are messed up ('f1' instead of 'title', 'f2' instead of 'review').
Questions:
How would the correct query be written?
Can you modify json_agg(sub2.*) to explicitly list the columns to aggregate? The most obvious way, e.g.json_agg((sub2.title, sub2.review)) creates json objects with nonsensical keys 'f1', 'f2', etc.
Is there a better way to convert object graphs to JSON? Correctly nesting queries, and grouping and aggregating fields is tricky to get right. How would you write a program that generates a SQL query returning JSON given an object graph?