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This article will explain in detail the operation examples of array types in MongoDB. The editor thinks it is very practical, so I share it with you for reference. I hope you can get something after reading this article.

In MongoDB schemas, we often store some data in array types, which is an implementation of our common nested pattern design. This design and implementation of arrays is not or is not common in relational databases. So, through this article, we will sort out the related operations of the array of MongoDB. Operations on arrays can be divided into two categories, one is array operators, the other is array operation modifiers.

Array operator

The operator implements the function $to locate the document $push to be updated according to the query selector to add the value to the array $pushAll to add the array to an array. (to be replaced by $rach) $addToSet

Add a value to the array and do not handle it if you repeat it

Pop removes the first or last value from the array. Pull removes values from the array that match the query criteria. PullAll removes multiple values from the array.

Array operation modifier

The modifier implements the function $each is used with $push and $addToSet to manipulate multiple values. $slice is used with $push and $each to reduce the size of the updated array. $sort sorts the subdocuments in the array along with $push, $each, and $slice.

1.$push operator

1.1 Grammar and function description

Push is mainly used to add elements to the array.

Syntax:

{$push: {:,...}}

By default, it adds a separate element to the end of the array.

1.2 Action case

Suppose we have a collection of student scores, studentscore, whose document format is as follows:

{"_ id": 1, "name": "xiaoming", "score": [{"math": 99, "english": 89}]} {"_ id": 2, "name": "xiaohong", "score": [{"math": 98, "english": 96}]}

The requirement is to update the document record with an id of 1, add the physics score to the field of the fractional array, and modify the code as

Db.studentscore.update ({_ id:1}, {$push: {score: {"physics": 100})

After modification, the result query is as follows:

{"_ id": 1, "name": "xiaoming", "score": [{"math": 99, "english": 89}, {"physics": 100}]} {"_ id": 2, "name": "xiaohong", "score": [{"math": 98, "english": 96}]} 1.3 combine the $each modifier to insert in batches

If you add more than one value to an array at a time, you can use it with the array modifier $each.

For example, we add the physics, chemistry and biology scores of Xiao Hong (_ id = 2) to the document. The statement executed is as follows:

Db.studentscore.update ({_ id: 2}, {$push: {score: {$each: [{"physics": 100}, {"chemistry": 90}, {"biology": 99}]}})

The query results are as follows:

{"_ id": 1, "name": "xiaoming", "score": [{"math": 99, "english": 89}, {"physics": 100}]} {"_ id": 2, "name": "xiaohong", "score": [{"math": 98, "english": 96}, {"physics": 100}, {"chemistry": 90} {"biology": 99}]} 1.4 use of array modifiers $sort and $slice

Having talked about the $each array operation modifier, let's give another example and explain the remaining two modifiers together ($sort and $slice).

For example, we have documentation as follows:

{"_ id": 5, "quizzes": [{"wk": 1, "score": 10}, {"wk": 2, "score": 8}, {"wk": 3, "score": 5}, {"wk": 4, "score": 6}]}

Now we have a requirement that we first append three records to the quizzes array field of the document, and then we sort by score to select the first three elements in the array.

Db.students.update ({_ id: 5}, {$push: {quizzes: {$each: [{wk: 5, score: 8}, {wk: 6, score: 7}, {wk: 7, score: 6}], $sort: {score:-1}, $slice: 3})

The updated results are shown as follows:

{"_ id": 5, "quizzes": [{"wk": 1, "score": 10}, {"wk": 2, "score": 8}, {"wk": 5, "score": 8}]}

The $slice operation modifier is added in MongoDB 2.4 to facilitate the management of frequently updated arrays. This operator is useful when you add values to an array but do not want the array to be too large. It must be used with the $push and $each operators to allow you to shorten the size of the array and delete old values.

Much like the $slice operation modifier, MongoDB 2.4 has added a $sort operation modifier to help update arrays. When using $push and $slice, it is sometimes necessary to sort them before deleting them.

2. $pop operator 2.1Syntax and function description

The $pop operator removes the first or best element from the array.

{$pop: {:,...}}

The parameter is-1, which represents the first element in the array to be deleted, and the parameter 1, which represents the last element in the array to be deleted.

2.2 Action case

For example, the following documents are found in the collection students:

{_ id: 1, scores: [8, 9, 10]}

Our requirement is to remove the first element (score 8) from the array, as shown in the SQL statement:

Db.students.update ({_ id: 1}, {$pop: {scores:-1}})

After the update, the document is as follows

{_ id: 1, scores: [9, 10]}

To continue the demonstration, if we need to further remove the last element of the array (score 10) on the existing basis, the updated sQL is as follows:

Db.students.update ({_ id: 1}, {$pop: {scores: 1}})

The query results are as follows:

{_ id: 1, scores: [9]} 3. $pull operator 3.1 syntax and function description

$pull is a complex form of $pop. With $pull, you can specify exactly which elements to delete by values.

Grammatical format

{$pull: {:,:,...} 3.2 Action case 3.2.1 removes elements from the array equal to the specified value

The test documents are as follows:

{_ id: 1, fruits: ["apples", "pears", "oranges", "grapes", "bananas"], vegetables: ["carrots", "celery", "squash", "carrots"]} {_ id: 2, fruits: ["plums", "kiwis", "oranges", "bananas", "apples"], vegetables: ["broccoli", "zucchini", "carrots" "onions"]}

The operation requires that the "apples" and "oranges" in the array field fruits be removed, and the "carrots" in the vegetables array field be removed with the following update statement:

Db.stores.update ({}, {$pull: {fruits: {$in: ["apples", "oranges"]}, vegetables: "carrots"}, {multi: true})

The updated results are as follows:

{"_ id": 1, "fruits": ["pears", "grapes", "bananas"], "vegetables": ["celery", "squash"]} {"_ id": 2, "fruits": ["plums", "kiwis", "bananas"], "vegetables": ["broccoli", "zucchini", "onions"]}

At this point, in the collection document, the array field of fruit has no apples, no oranges,vegetables array field, and no carrots.

3.2.2 remove elements from the array that meet the specified criteria

Suppose we have a collection of profiles whose document format is as follows:

{_ id: 1, votes: [3, 5, 6, 7, 7, 8]}

We want to remove the element whose votes is greater than or equal to 6 with the following statement:

Db.profiles.update ({_ id: 1}, {$pull: {votes: {$gte: 6})

The updated results are as follows:

{_ id: 1, votes: [3,5]} 3.2.3 remove embedded subdocuments from the array (that is, the array elements are subdocuments, and the content in each {} is an array element)

Suppose we have a collection of surveys called survey, whose data are as follows:

{_ id: 1, results: [{item: "A", score: 5}, {item: "B", score: 8, comment: "Strongly agree"}]} {_ id: 2, results: [{item: "C", score: 8, comment: "Strongly agree"}, {item: "B", score: 4}]}

The requirement is to remove the element whose score is 8 and item is "B"

Db.survey.update ({}, {$pull: {results: {score: 8, item: "B"}, {multi: true})

The updated document is as follows:

{"_ id": 1, "results": [{"item": "A", "score": 5}]} {"_ id": 2, "results": [{"item": "C", "score": 8, "comment": "Strongly agree"}, {"item": "B" "score": 4}]} 3.2.4 if elements of array type are also embedded in an array (array package array) You have to be very careful.

The $elemMatch operator will be used at this point.

For example, the document format is as follows:

{_ id: 1, results: [{item: "A", score: 5, answers: [{Q: 1, a: 4}, {Q: 2, a: 6}]}, {item: "B", score: 8, answers: [{Q: 1, a: 8}, {Q: 2, a: 9}]} {_ id: 2, results: [{item: "C" Score: 8, answers: [{Q: 1, a: 8}, {Q: 2, a: 7}]}, {item: "B", score: 4, answers: [{Q: 1, a: 0}, {Q: 2, a: 8}]}

The results array field needs to be removed if the answers field in the results array field conforms to a Q of 2 and a greater than or equal to 8.

Db.survey.update ({}, {$pull: {results: {answers: {$elemMatch: {Q: 2, a: {$gte: 8}}, {multi: true})

The updated data are as follows:

{"_ id": 1, "results": [{"item": "A", "score": 5, "answers": [{"Q": 1, "a": 4}, {"Q": 2, "a": 6}]}} {"_ id": 2, "results": [{"item": "C" "score": 8, "answers": [{"Q": 1, "a": 8}, {"Q": 2, "a": 7}]} 4.$addToSet4.1 grammar and function description

Using $addToSet also adds values to the end of the array, but it's special: it only adds values that don't exist in the array.

{$addToSet: {:,...} 4.2 Operation case

If you have a collection in the following inventory format

{_ id: 1, item: "polarizing_filter", tags: ["electronics", "camera"]}

If we want to add an element accessories to the field tags array, the update statement is as follows:

Db.inventory.update ({_ id: 1}, {$addToSet: {tags: "accessories"}})

The updated result is

{"_ id": 1, "item": "polarizing_filter", "tags": ["electronics", "camera", "accessories"]}

If you want to add elements in batches, we can use them in conjunction with the $each operator.

For example, the following documents

{_ id: 2, item: "cable", tags: ["electronics", "supplies"]}

If we want to add the elements "camera", "electronics", "accessories" to the field tags array, the update statement is as follows:

Db.inventory.update ({_ id: 2}, {$addToSet: {tags: {$each: ["camera", "electronics", "accessories"]})

The updated results are as follows:

{_ id: 2, item: "cable", tags: ["electronics", "supplies", "camera", "accessories"]} 4.3 attention points

It should be noted that if the added element is in an array format, the newly added element will be retained as an array (an array nested array will appear)

For example

{_ id: 1, letters: ["a", "b"]}

The statement executed is as follows:

Db.test.update ({_ id: 1}, {$addToSet: {letters: ["c", "d"]}})

The query structure is displayed as

{_ id: 1, letters: ["a", "b", ["c", "d"]} examples of array types in MongoDB are shared here. I hope the above can be helpful and learn more. If you think the article is good, you can share it for more people to see.

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