Using MongoDB

Specify Conditions with Operators

Operators

MongoDB provides operators to specify query conditions, such as comparison operators. Although there are some exceptions, such as the $or conditional operator, query conditions using operators generally have the following form:

{ <field1>: { <operator1>: <value1> } }

Let’s first define a custom helper object to reduce the boilerplate necessary for “testing” a query document by projecting a few fields for inspection.

my_proj = {material_id: 1, pretty_formula: 1, _id: 0}

Because MongoDB queries and projections are expressed as JSON, one can programmatically build them without resorting to string manipulation.

Let’s find some materials with at least three elements. At the same time, we’ll demonstrate a simple example of “building” a projection document on-the-fly. In this case, because we’re in the mongo shell, we use the Javascript language. Object.assign takes a “destination” object as its first argument, successively merges the key-value pairs of subsequently provided “source” objects into the destination object (overwriting if necessary), and returns the destination object.

db.materials.find({nelements: {$gte: 3}}, Object.assign({nelements: 1}, my_proj))

{ "nelements" : 3, "pretty_formula" : "La2SiO5", "material_id" : "mp-5152" }
{ "nelements" : 3, "pretty_formula" : "Er2SO2", "material_id" : "mp-12671" }
{ "nelements" : 3, "pretty_formula" : "Fe4O7F", "material_id" : "mp-780541" }
...

And some with fewer than (“less than”) 3:

db.materials.find({nelements: {$lt: 3}}, Object.assign({nelements: 1}, my_proj))

{ "nelements" : 1, "pretty_formula" : "Fe", "material_id" : "mp-568345" }
{ "nelements" : 2, "pretty_formula" : "YbZn", "material_id" : "mp-1703" }
{ "nelements" : 2, "pretty_formula" : "LuB6", "material_id" : "mp-12660" }
...

Combining Conditions

You can specify a logical conjunction (AND) for a list of query conditions by separating the conditions with a comma in the conditions document:

db.materials.find({chemsys: "Fe-O", "spacegroup.crystal_system": "cubic"}).count()

7

You can specify a logical disjunction (OR) for a list of query conditions by using the $or query operator.

db.materials.find({
    $or: [{nelements: 2}, {nelements: 4}]
}).count()

27370

db.materials.find({elasticity: {$exists: true}})

{elasticity: {$ne: null}}