Reading from primary and secondary changing the order of the documents of the same query

[Aayushi_Mangal]

Hi,

I am executing a query from Primary and then Secondary, the order (sequence of the result is different).
When I am executing a query using _id sort, then the order is same.
I there anyway we can have same query results without doing sorting on _id field.

[Kobe_W]

What is your query like?

[Aayushi_Mangal]

Hi Kobe,

these is my sample query:

mongos> db.products.find().readPref(“primary”)
{ “_id” : 10, “item” : “1”, “qty” : 20 }
{ “_id” : 11, “item” : “2”, “qty” : 20 }
{ “_id” : 12, “item” : “3”, “qty” : 20 }
mongos> db.products.find().readPref(“secondary”)
{ “_id” : 10, “item” : “1”, “qty” : 20 }
{ “_id” : 12, “item” : “3”, “qty” : 20 }
{ “_id” : 11, “item” : “2”, “qty” : 20 }

Here orders getting changed

[Kobe_W]

If no sort condition is specified, natural ordering is used by mongodb.

cursor.sort() — MongoDB Manual

This is internal implementation so relying on this order is almost always bad practice. You should explicitly use a sort (e.g. on _id). Otherwise the order of returned docs is nondeterministic.