How to select query from json in Postgres - json

Question

I have JSON data in field hotel_data like this:
{
"title":"foo",
"description":[
{
"locale":"pt",
"content":"pt text"
},
{
"locale":"fr",
"content":"fr text"
}
]
}
I would like to select description only fr description. It is possible using Postgres and how?
I was trying use ->> but it is not working...
SELECT
hotel_data->'description'->>'locale' = 'fr' AS description
FROM hotel LIMIT 1;
Note:
I don't want to use SELECT *...
Excepted output: {description: "fr text"}

Answer 1

You can use a lateral join and json_to_recordset to expand the json array as a set of records. Then, you can filter on column locale in the generated records, and finally recompose a new json object with your expected result:
select json_build_object('description', d.content) hotel_data_descr_fr
from
mytable,
json_to_recordset(hotel_data->'description') as d("locale" text, "content" text)
where d.locale = 'fr'
Demo on DB Fiddle:
with mytable as (
select '{
"title":"foo",
"description":[
{
"locale":"pt",
"content":"pt text"
},
{
"locale":"fr",
"content":"fr text"
}
]
}'::json hotel_data
)
select json_build_object('description', d.content) hotel_data_descr_fr
from
mytable,
json_to_recordset(hotel_data->'description') as d("locale" text, "content" text)
where d.locale = 'fr'
| hotel_data_descr_fr |
| :------------------------- |
| {"description": "fr text"} |

Answer 2

The filtering can be done using the #> operator which can use a GIN index on the hotel_data column. This is typically faster than expanding the array.
select ...
from hotel
where hotel_data #> '{"description": [{"locale":"fr"}] }';
This can also be extended to include more properties:
select ...
from hotel
where hotel_data #> '{"description": [{"locale":"fr", "headline": "nice view'}] }';
But you can only express equality conditions on the key/value pairs with that. Using LIKE is not possible. You will have to expand the array if you want to do that and apply the condition in the WHERE clause - see GMB's answer.
To extract that description, I would use a scalar sub-query:
select (select jsonb_build_object('description', t.descr ->> 'content')
from jsonb_array_elements(h.hotel_data -> 'description') as t(descr)
where t.descr ->> 'locale' = 'fr'
limit 1)
from hotel h
where h.hotel_data #> '{"description": [{"locale":"fr"}] }';
That way you don't need to expand the array for filtering which I expect to be faster if only a few hotels qualify for that condition. But it has the drawback that you need to repeat the condition on the locale in the sub-select.
The limit 1 is only a safety net in case you have more than one french description. If you never have that, it doesn't hurt either
With Postgres 12 this is easier:
select jsonb_build_object(
'description',
jsonb_path_query_first(hotel_data, '$.description ? (#.locale == "fr")') -> 'content'
)
from hotel
where hotel_data #> '{"description": [{"locale":"fr"}] }'
All of the above assumes hotel_data is a jsonb column, if it's not (which it should be) you need to cast it: hotel_data::jsonb