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3+ < img src ="/docs/images/pencil.svg " />
4+ </ a > < h1 id ="queries-in-typescript ">
5+ < a href ="#queries-in-typescript ">
6+ Queries in TypeScript
7+ </ a >
8+ </ h1 >
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14+
15+ < p > Mongoose's < a href ="../api/query.html "> Query class</ a > is a chainable query builder that represents a MongoDB query.
16+ When you call < code > find()</ code > , < code > findOne()</ code > , < code > updateOne()</ code > , < code > findOneAndUpdate()</ code > , etc. on a model, Mongoose will return a Query instance.
17+ Queries have a < code > .then()</ code > function that returns a Promise, so you can use them with < code > await</ code > .</ p >
18+ < p > In TypeScript, the Query class takes the following generic parameters:</ p >
19+ < pre > < code lang ="ts "> < span class ="hljs-keyword "> class</ span > < span class ="hljs-title class_ "> Query</ span > <
20+ < span class ="hljs-title class_ "> ResultType</ span > , < span class ="hljs-comment "> // The type of the result of the query, like `DocType[]`</ span >
21+ < span class ="hljs-title class_ "> DocType</ span > , < span class ="hljs-comment "> // The hydrated document type of the query's associated model</ span >
22+ < span class ="hljs-title class_ "> THelpers</ span > = {}, < span class ="hljs-comment "> // Query helpers</ span >
23+ < span class ="hljs-title class_ "> RawDocType</ span > = < span class ="hljs-built_in "> unknown</ span > , < span class ="hljs-comment "> // The "lean" document type of the query's associated model</ span >
24+ < span class ="hljs-title class_ "> QueryOp</ span > = < span class ="hljs-string "> 'find'</ span > , < span class ="hljs-comment "> // The operation that will be executed, like 'find', 'findOne', 'updateOne', etc.</ span >
25+ < span class ="hljs-title class_ "> TDocOverrides</ span > = < span class ="hljs-title class_ "> Record</ span > << span class ="hljs-built_in "> string</ span > , < span class ="hljs-built_in "> never</ span > > < span class ="hljs-comment "> // Methods and virtuals on the hydrated document</ span >
26+ ></ code > </ pre > < h2 id ="using-lean-in-typescript ">
27+ < a href ="#using-lean-in-typescript ">
28+ Using < code > lean()</ code > in TypeScript
29+ </ a >
30+ </ h2 >
31+ < p > The < a href ="../tutorials/lean.html "> < code > lean()</ code > method</ a > tells Mongoose to skip < a href ="../api/model.html#model_Model-hydrate "> hydrating</ a > the result documents, making queries faster and more memory efficient.
32+ < code > lean()</ code > comes with some caveats in TypeScript when working with the query < code > transform()</ code > function.
33+ In general, we recommend calling < code > lean()</ code > before using the < code > transform()</ code > function to ensure accurate types.</ p >
34+ < pre > < code lang ="ts "> < span class ="hljs-comment "> // Put `lean()` **before** `transform()` in TypeScript because `transform` modifies the query ResultType into a shape</ span >
35+ < span class ="hljs-comment "> // that `lean()` does not know how to handle.</ span >
36+ < span class ="hljs-keyword "> const</ span > result = < span class ="hljs-keyword "> await</ span > < span class ="hljs-title class_ "> ProjectModel</ span >
37+ .< span class ="hljs-title function_ "> find</ span > ()
38+ .< span class ="hljs-title function_ "> lean</ span > ()
39+ .< span class ="hljs-title function_ "> transform</ span > (< span class ="hljs-function "> (< span class ="hljs-params "> docs</ span > ) =></ span > < span class ="hljs-keyword "> new</ span > < span class ="hljs-title class_ "> Map</ span > (docs.< span class ="hljs-title function_ "> map</ span > (< span class ="hljs-function "> (< span class ="hljs-params "> doc</ span > ) =></ span > [doc.< span class ="hljs-property "> _id</ span > .< span class ="hljs-title function_ "> toString</ span > (), doc])));
40+
41+ < span class ="hljs-comment "> // Do **not** do the following</ span >
42+ < span class ="hljs-keyword "> const</ span > result = < span class ="hljs-keyword "> await</ span > < span class ="hljs-title class_ "> ProjectModel</ span >
43+ .< span class ="hljs-title function_ "> find</ span > ()
44+ .< span class ="hljs-title function_ "> transform</ span > (< span class ="hljs-function "> (< span class ="hljs-params "> docs</ span > ) =></ span > < span class ="hljs-keyword "> new</ span > < span class ="hljs-title class_ "> Map</ span > (docs.< span class ="hljs-title function_ "> map</ span > (< span class ="hljs-function "> (< span class ="hljs-params "> doc</ span > ) =></ span > [doc.< span class ="hljs-property "> _id</ span > .< span class ="hljs-title function_ "> toString</ span > (), doc])))
45+ .< span class ="hljs-title function_ "> lean</ span > ();</ code > </ pre > < p > In general, if you're having trouble with < code > lean()</ code > inferring the correct type, you can try moving < code > lean()</ code > earlier in the query chain.</ p >
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