Is it possible to add a constraint to check every inserted object into a JSON array using Postgres? - arrays

Question

I am trying to figure out how to add a constraint to my table that contains a JSON object in Postgres. I would like that constraint to make sure a jpg will have a non null md5 property.
I have tried to check the subarray but I cannot get it to work on an array. I can test the property if use the dereferencing arrow operator (table->'jpg'->0 ? 'md5'). I have tried to use the jsonb_array_elements() function as well but it is not allowed inside a constraint statement.
For instance:
{
jpg: [
{
md5: "some md5",
...
}
]
}
I would use this:
ALTER TABLE table ADD CONSTRAINT md5_is_defined CHECK(table->'jpg'->0 ? 'md5')
But I don't want to check just the first inserted element
I would like to know if there is some trick to use jsonb_array_elements in this check statement. And yes I realize I should normalize my data. I tried to do this but it became a monstrosity to write the statements for.

Answer 1

Arrays are extremely cumbersome to deal with in SQL as they basically violate anything that SQL does. Checking every element in an array for the presence of something is typically a strong indication that an array was the wrong choice to begin with.
With Postgres 12 this is extremely easy to do:
alter table the_table
add constraint md5_is_defined
check (jsonb_path_exists(the_column, '$.jpg[*].md5'));
For older versions, the only thing I can think of is to create a function that checks the presence and then use that function in the check constraint:
create or replace function check_md5(p_input jsonb)
returns boolean
as
$$
select exists (select *
from jsonb_array_elements(p_input -> 'jpg') as t(e)
where e ? 'md5');
$$
language sql
immutable;
Then you can use it like this:
alter table the_table
add constraint md5_is_defined
check (check_md5(the_column));
Edit
To check if a specific key contains a non-empty array you can use something like this:
alter table the_table
add constraint non_empty_array
check (jsonb_path_exists(the_column, '$.event ? (#.teams.type() == "array" && #.teams.size() > 0)'));
The check for #.teams.type() == "array" is necessary because a simple {"teams": "yes"} also returns a non-zero value for size()