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How do I query elements in an array in MongoDB? I believe that many inexperienced people are at a loss about this, so this article summarizes the causes and solutions of the problem. Through this article, I hope you can solve this problem.

Preface

MongoDB is a document database, and each doc represents a record of the data. Whereas relational DB's row can only use simple data types, doc can use complex data types: embedded doc, arrays. The array of MongoDB is a collection of elements, using parentheses [] to indicate the array. For example, the element of [1 name: "T5"}, {name: "T7"}], [{name: "T5", age:21}, {name: "T7", age:22}] is an integer value.

In MongoDB, array elements are allowed to repeat, and the position of the element is fixed. If two arrays are equal, then the elements and their positions of both arrays are the same.

There are two ways to match based on an array of child elements in MongoDB.

Match using "[array name]. [child element field name]".

Use the method of "[array name]" $elemMatch {[child element field name]}.

The difference lies in the matching subject.

The matching body of "[array name]. [child element field name]" is "[array name]", which is suitable for a single condition, and if it is multiple conditions, it becomes an "OR" operation between the child elements of the array.

Take a look at the example:

Suppose there are two pieces of data in a collection:

Document1 is as follows:

{"_ id": "name": "Humanities Medicine", "qList": [{"qid": 1, "content": "principles of Justice in Medical Ethics", "reorderFlag": 1}, {"qid": 2, "content": "formulating basic principles for Human experiments", "reorderFlag": 0}]}

Document2 is as follows:

{"_ id": "name": "Humanities Medicine 2", "qList": [{"qid": 1, "content": "principles of Justice in Medical Ethics", "reorderFlag": 0}, {"qid": 2, "content": "formulating basic principles for Human experiments", "reorderFlag": 1}]}

Find the records in the array that have qid=1 and reorderFlag=0

Query statements in an array that meet two conditions for the same record:

{"qList": {$elemMatch: {"qid": 1, "reorderFlag": 0}}}

The query results are as follows:

{"_ id": "name": "Humanities Medicine 2", "qList": [{"qid": NumberInt (1), "content": "principles of Justice in Medical Ethics", "reorderFlag": NumberInt (0)}, {"qid": NumberInt (2), "content": "formulating basic principles for Human experiments" "reorderFlag": NumberInt (1)}]}

As you can see, the result of its execution is that $elemMatch matching is performed on each child element in the array, and multiple conditions can be matched.

Find the records of qid=1 or reorderFlag=0 in the array

The array as a whole can meet the following two conditions:

{"qList.qid": 1, "qList.reorderFlag": 0}

The main body of the execution is qList, which requires that some child elements satisfy qid=1 and some child elements satisfy reorderFlag= 0`.

The query results are as follows:

{"_ id": "name": "Humanities Medicine", "qList": [{"qid": NumberInt (1), "content": "principles of Justice in Medical Ethics", "reorderFlag": NumberInt (1)}, {"qid": NumberInt (2), "content": "formulating basic principles for Human experiments" "reorderFlag": NumberInt (0)}} {"_ id", "name": "Humanities Medicine 2", "qList": [{"qid": NumberInt (1), "content": "principles of Justice in Medical Ethics", "reorderFlag": NumberInt (0)}, {"qid": NumberInt (2) "content": "formulating basic principles for human experiments", "reorderFlag": NumberInt (1)}]}

As you can see, the result of its execution is to match the array, where the child elements need to satisfy "qList.qid": 1, and the child elements need to meet "qList.qid": 1, which is suitable for matching a single condition.

If a single condition matches, the result is the same in the following ways.

{"qList.qid": 1}

Or

{"qList": {$elemMatch: {"qid": 1}}}

The results of the query are all 2 records.

After reading the above, have you mastered how to query the elements in the array in MongoDB? If you want to learn more skills or want to know more about it, you are welcome to follow the industry information channel, thank you for reading!

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