SQL query about to bind multiple elements to a specific one - json

Question

I am going to convert my current PostgreSQL database into a MongoDB version. For example, I have a table to record tweets, and another table to record multiple hashtags used by a specific tweet. What I wanna do is to use SQL to get a table like below and then export it as a .csv file so that I could import it to MongoDB.
Example:
2018-04-02 18:12:32 This plane has no outlet for me to charge my p... [{'tag': 'GucciGarden', 'airline': 'American A...
The problem that I met is that I can get a .csv file contains json array like "[{'tag': 'GucciGarden', 'airline': 'American A...", but it is a String type! And when I import it into MongoDB. The quote will be kept, which makes sth wrong.
And here is my SQL code:
SELECT tweets.tweet_id,tweets.text,
(SELECT array_to_json(array_agg(row_to_json(d)))
from (
SELECT tags.tag
FROM
tags
WHERE tags.tweet_id=tweets.tweet_id
) d
) as Tags
from tweets
Here is the result that I import into MongoDB:
{
"_id" : ObjectId("5ac59c272221ade1185ec241"),
"tweet_id" : 9.80869021435351e+17.0,
"created_at" : "2018-04-02 18:06:13",
"text" : "RT #MiraSorvino: Brad Myles shares #Delta that awareness is working- 9,000 #humantrafficking cases identified by #polarisproject National H��",
"screen_name" : "MMexville",
"favorite_count" : 0.0,
"retweet_count" : 40.0,
"source" : "the public",
"tags" : "[{'tag': 'humantrafficking', 'airline': 'Delta Air Lines'}]"}

Answer 1

this is because [{'tag': is not a valid json - you should have used double quotes and cast to json, eg:
let's say smth like your sample:
t=# create table c (i int, t text, j text);
CREATE TABLE
t=# insert into c values(1,'text',$$[{'tag': 'GucciGarden'}]$$);
INSERT 0 1
t=# select * from c;
i | t | j
---+------+--------------------------
1 | text | [{'tag': 'GucciGarden'}]
(1 row)
so then smth like your qry:
t=# select to_json(c) from (select i,t,replace(j,$$'$$,'"')::json j from c) c;
to_json
-------------------------------------------------
{"i":1,"t":"text","j":[{"tag": "GucciGarden"}]}
(1 row)
of course you will have positive false replacements of single quotes, eg 'tag': 'Gucci's Garden' will break the query logic, so you will have to make a more sophisticated replacement. probably with regular expressions to be neater.