unique index on embedded json object

Hi forks,
I am testing postgresql 9.4 beta2 right now. I am wondering if it is possible to create a unique index on embedded json object?

For example,

I create a table names "products"

CREATE TABLE products (oid serial primary key, data jsonb)
Now, I try to insert json object into data column.
{"id": "12345","bags": [{ "sku": "abc123", "price": 0,},{ "sku": "abc123", "price": 0,}]}
However, I want sku of bags to be unique. It means the json can't be inserted into products tables because sku is not unique in this case.
I tried to create a unique index like below, but it failed.
CREATE UNIQUE INDEX product_sku_index ON products( (data->'bags'->'sku') )
Any suggestions, please? Thank you.

Thanks,
Jason