How can I achieve mongo "unwind" in postgres JSONB? (Flatten nested arrays) - mongodb

Question

I recently looked into migrating our product database from mongo to postgres. Coming from mongoDb, I am used of "unwinding" objects and arrays.
Suppose you have the following object:
{
"styleGroupId": "2",
"brand": "MOP",
"colorVariants": [
{
"color": "red",
"colorCode": "222",
"sizeVariants": [
{"gtin": "444",
"size": "M"},
{"gtin": "555",
"size": "L"}
]
},
{
"color": "blue",
"colorCode": "111",
"sizeVariants": [
{"gtin": "66",
"size": "M"},
{"gtin": "77",
"size": "L"}
]
}
]
}
If you want to flatten it, in mongo you use the following:
db.test.aggregate([
{
$unwind: "$colorVariants"
},
{
$unwind: "$colorVariants.sizeVariants"
}
])
which will result in objects like this:
{
"_id" : ObjectId("5a7dc59dafc86d25964b873c"),
"styleGroupId" : "2",
"brand" : "MOP",
"colorVariants" : {
"color" : "red",
"colorCode" : "222",
"sizeVariants" : {
"gtin" : "444",
"size" : "M"
}
}
}
I have spent hours searching for "mongo unwind in postgres" but could hardly find a satisfying answer. Also a lot of the resources on querying JSONB data in postgres barely touch nested arrays. Hopefully this post will help other poor souls searching for a migration from mongoDb to postgres.

Answer 1

The function:
create or replace function jsonb_unwind(target jsonb, path text[])
returns jsonb language plpgsql as $$
begin
if jsonb_typeof(target #> path) = 'array' then
return jsonb_set(target, path, target #> path -> 0);
else
return target;
end if;
end $$;
Example usage:
select
jsonb_unwind(
jsonb_unwind(json_data, '{colorVariants}'),
'{colorVariants, sizeVariants}')
from my_table;
Test it in rextester.

Answer 2

The answer could implicitly found in this post: How to query nested arrays in a postgres json column?
edit
paired with this answer use - to remove a key from a JSONB
there is an easier way to get sth closer to mongo unwind starting from 9.6
You need to state each array level in the FROM clause of your query
exclude each nested object from your output so it doesn't appear multiple times
optional concatenate object and subobject
when concatenating objects with shared keys, only the key of the object right of the || operator will be preserved
-
SELECT
data::jsonb - 'colorVariants' ||
colorVariants.* - 'sizeVariants' ||
sizeVariants.*
FROM
test,
jsonb_array_elements(data -> 'colorVariants') colorVariants,
jsonb_array_elements(colorVariants -> 'sizeVariants') sizeVariants;
the result then is
?column?
-------------------------------------------------------------------------------------------------------
{"gtin": "11", "size": "M", "brand": "MOP", "color": "red", "colorCode": "222", "styleGroupId": "1"}
{"gtin": "22", "size": "L", "brand": "MOP", "color": "red", "colorCode": "222", "styleGroupId": "1"}
{"gtin": "33", "size": "M", "brand": "MOP", "color": "blue", "colorCode": "111", "styleGroupId": "1"}
{"gtin": "44", "size": "L", "brand": "MOP", "color": "blue", "colorCode": "111", "styleGroupId": "1"}
{"gtin": "444", "size": "M", "brand": "MOP", "color": "red", "colorCode": "222", "styleGroupId": "2"}
{"gtin": "555", "size": "L", "brand": "MOP", "color": "red", "colorCode": "222", "styleGroupId": "2"}
{"gtin": "66", "size": "M", "brand": "MOP", "color": "blue", "colorCode": "111", "styleGroupId": "2"}
{"gtin": "77", "size": "L", "brand": "MOP", "color": "blue", "colorCode": "111", "styleGroupId": "2"}
old post
You have to state each array level in the FROM clause of your query
You have to specifically list the attributes of each level. If you only state e.g. colorVariants the nested arrays will also be returned
So in the specific case the solution would be:
SELECT
data->'brand' as brand,
data->'styleGroupId' as styleGroupId,
colorVariants->'color' as color,
colorVariants->'colorCode' as colorCode,
sizeVariants->'gtin' as GTIN,
sizeVariants->'size' as size
FROM
test,
jsonb_array_elements(data->'colorVariants') colorVariants,
jsonb_array_elements(colorVariants->'sizeVariants') sizeVariants
which will result in
brand | stylegroupid | color | colorcode | gtin | size
-------+--------------+--------+-----------+-------+------
"MOP" | "1" | "red" | "222" | "11" | "M"
"MOP" | "1" | "red" | "222" | "22" | "L"
"MOP" | "1" | "blue" | "111" | "33" | "M"
"MOP" | "1" | "blue" | "111" | "44" | "L"
"MOP" | "2" | "red" | "222" | "444" | "M"
"MOP" | "2" | "red" | "222" | "555" | "L"
"MOP" | "2" | "blue" | "111" | "66" | "M"
"MOP" | "2" | "blue" | "111" | "77" | "L"

Answer 3

Very simple example.
Suppose you have the following data in userGroups table
id | userId | groups
1 | 21 | [{"name": "group_1" }, { "name": "group_2" }]
if you query for attributes with json_array_elements function for groups attribute
select id, userId, json_array_elements(groups) from userGroups;
you will get results similar to mongodb unwind operator results
id | userId | groups
1 | 21 | {"name": "group_1" }
1 | 21 | {"name": "group_2" }
Hope it helps someone!