Find data

KMongo operators primarily work by accepting varargs which are combined into Bson documents.

KtMongo operators primarily work by exposing a DSL in which operators can be bound.

For example,

Using KMongo

collection.findOne(
    and(
        User::name.exists(),
        User::age gt 18,
    )
)

becomes:

Using KtMongo

collection.findOne {
    and {
        User::name.exists()
        User::age gt 18
    }
}

Note how:

• the parentheses become braces,
• the trailing commas are gone.

Default composition operators

Each operation has a default operator when multiple values are passed. For example, findOne has a default of $and, meaning that these two snippets are identical:

collection.findOne {
    and {
        User::name.exists()
        User::age gt 18
    }
}

collection.findOne {
    User::name.exists()
    User::age gt 18
}

Complex requests

One advantage of the DSL syntax is using conditionals and loops directly into the request itself, making complex requests much easier to write.

Here's an example of a complex request with KMongo:

Using KMongo

val bson = ArrayList<Bson>()

if (criteria.name != null)
    bson.add(User::name eq criteria.name)

if (criteria.age != null)
    bson.add(User::age eq criteria.age)

collection.findOne(and(bson))

When these kinds of requests grow, they become harder to understand because the criteria are defined further from the operation call.

With KtMongo, everything is co-located and the intermediate list is eliminated:

Using KtMongo

collection.findOne {
    if (criteria.name != null)
        User::name eq criteria.name

    if (criteria.age != null)
        User::age eq criteria.age
}

Since the specific use-case of optional filter criteria is so common, KtMongo offers specific operators that only apply to the request when their argument is non-null:

Using KtMongo

collection.findOne {
    User::name eqNotNull criteria.name
    User::age eqNotNull criteria.age
}