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CURD
Query

Query

Find

The find() method is used to query a collection for documents that match specific query criteria. When called with no arguments, the find() method returns a cursor pointing to the results. You can manipulate the cursor to retrieve the data from the database.

#[derive(Debug, Serialize, Deserialize)]
struct Book {
   title: String,
   author: String,
}
 
let db = Database::open_memory()?;
let collection = db.collection::<Book>("books");
 
let docs = vec![
    Book { title: "1984".to_string(), author: "George Orwell".to_string() },
    Book { title: "Animal Farm".to_string(), author: "George Orwell".to_string() },
    Book { title: "The Great Gatsby".to_string(), author: "F. Scott Fitzgerald".to_string() },
];
collection.insert_many(docs)?;
 
let books = collection.find(None)?;
for book in books {
    println!("name: {:?}", book);
}

Example:

If we want to find a book written by George Orwell, we can find using the author's name like this:

let books = collection.find(doc! {
    "author": "George Orwell",
})?;
 
for book in books {
    println!("name: {:?}", book);
}

If you only want to find the first result match the filter, use find_one:

let book = collection.find_one(doc! {
    "author": "George Orwell",
})?;
 
println!("name: {:?}", book);

Specify AND Conditions

A compond query can specify conditions for more than one field in the collection's documents. Implicit AND conjunction of the query clauses.

collection.find(doc! {
    "author": "George Orwell",
    "title": "1984",
})?;

Specify OR Conditions

Using the $or operator, you can specify a compound query that joins each clause with a logical OR conjunction so that the query selects the documents in the collection that match at least one condition.

let result = typed_collection
    .find(doc! {
        "$or": [
            {
                "title": "The Grapes of Wrath",
            },
            {
                "title": "To Kill a Mockingbird",
            }
        ]
    })?
    .collect::<Result<Vec<Document>>>()?;

Advanced conditions

NameDescription
$eqMatches values that are equal to a specified value.
$gtMatches values that are greater than a specified value.
$gteMatches values that are greater than or equal to a specified value.
$inMatches any of the values specified in an array.
$ltMatches values that are less than a specified value.
$lteMatches values that are less than or equal to a specified value.
$neMatches all values that are not equal to a specified value.
$ninMatches none of the values specified in an array.
$regexMatches values that are equal to the given regular expression.

Specify EQ and NE Condition

collection.find(doc! {
    "age": { "$eq": 18 },
})?;

The above query is specifying the age field must be equal to 18.

collection.find(doc! {
    "age": { "$ne": 18 },
})?;

While this one should is specifying the age field should not be equal to 18.

Specify GT and GTE Conditions

collection.find(doc! {
    "age": { "$gt": 18 },
})?;

The above query is specifying that the age field must be greater than 18.

collection.find(doc! {
    "age": { "$gte": 18 },
})?;

While this one is specifying the age field must be greater or equal to 18

Specify IN and NIN Conditions

collection.find(doc! {
    "age": { "$in": [18, 19, 20] },
})?;

The above query is specifying the age field must in the provided int slice.

collection.find(doc! {
    "age": { "$nin": [18, 19, 20] },
})?;

While this one is specifying the age field must not be equal to one of the values inside the int slice.

Specify LT and LTE Conditions

collection.find(doc! {
    "age": { "$lt": 18 },
})?;

The above query is specifying that the age field must be less than 18.

collection.find(doc! {
    "age": { "$lte": 18 },
})?;

While this one is specifying the age field must be less or equal to 18

Specify REGEX Condition

collection.find(doc! {
    "title": { "regex": polodb_core::bson::Regex {
        "pattern": "\w*Ring\w*",
        "options": "i",
    }}
})?;

The above query is specifying that the title field should match the given regular expression with its options.

Last updated on May 29, 2023
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