Using MongoDB

Sort Results

Learning Objectives

• Understand sorting as a method chained to the cursor
• Sorting by multiple keys

To specify an order for the result set, you can append the sort() method to a cursor. For ascending-order sorting by a single key of interest:

filt = {elasticity: {$ne: null}}
proj = {"elasticity.poisson_ratio": 1}
Object.assign(proj, my_proj)
db.materials.find(filt, proj).sort({"elasticity.poisson_ratio": 1})

{ "pretty_formula" : "WO3", "material_id" : "mp-771798", "elasticity" : { "poisson_ratio" : -0.07595596751510682 } }
{ "pretty_formula" : "Be", "material_id" : "mp-87", "elasticity" : { "poisson_ratio" : 0.042582069532848744 } }
{ "pretty_formula" : "MnCoO4", "material_id" : "mp-765892", "elasticity" : { "poisson_ratio" : 0.05989488523534276 } }
...

To specify descending-order sorting by a key:

db.materials.find(filt, proj).sort({"elasticity.poisson_ratio": -1}).pretty()

{
    "pretty_formula" : "AlV3",
    "material_id" : "mp-1387",
    "elasticity" : {
        "poisson_ratio" : 0.46752282089107655
    }
}
{
    "pretty_formula" : "Cu2O",
    "material_id" : "mp-361",
    "elasticity" : {
        "poisson_ratio" : 0.46347893160099135
    }
}
...

I also chose to chain pretty() on the cursor to pretty-print the results, i.e. auto-indent things. To turn pretty-printing on for this shell session, we can set

DBQuery.prototype._prettyShell = true

What happens if we want secondary sorting? The argument to sort() is still an object. The first key-value pair specifies the primary sorting, the second pair the secondary sorting after the first is done, and so on.

db.materials.find(filt, proj).sort(
    {nelements: -1, "elasticity.poisson_ratio": -1}
)

{
    "pretty_formula" : "SrSbSe2F",
    "material_id" : "mp-556194",
    "elasticity" : {
        "poisson_ratio" : 0.3419981312210572
    }
}
{
    "pretty_formula" : "KAg2PS4",
    "material_id" : "mp-12532",
    "elasticity" : {
        "poisson_ratio" : 0.34027796672596883
    }
}
...