Queries
Easily build Go queries in ObjectBox with the ObjectBox builder APIs. ObjectBox queries help you quickly find objects matching criteria you've specified.
Using queries is simple: from your entity's
Box, call Query() with conditions as arguments:1
query := box.Query(Device_.Location.HasPrefix("US-", false))
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devices, err := query.Find()
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Building queries
The query in the code above uses a function
HasPrefix on a device location. Where does this come from? ObjectBox generates a Device_ struct for you to reference available properties conveniently. This also allows code completion in your IDE and avoids typos: correctness is checked at compile time (string based queries would only be checked at run-time).Let's say you have the following entity defined in your package:
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type Device struct {
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Id uint64
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Name string
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Location string
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Profile uint32
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}
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Using this input, the ObjectBox code generator creates a variable
Device_ in the same package:1
var Device_ = struct {
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Id *objectbox.PropertyUint64
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Name *objectbox.PropertyString
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Location *objectbox.PropertyString
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Profile *objectbox.PropertyUint32
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}{...}
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You can use
Device_ to construct type-specific conditions in place and combining them, forming the full query. The following example looks for devices located in the U. S. with profile number 42.1
box.Query(Device_.Profile.Equals(42), Device_.Location.HasPrefix("US-", false))
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Reusing Queries and Parameters
If you frequently run a
Query you should cache the Query object and re-use it. To make a Query more reusable you can change the values, or query parameters, of each condition you added even after the Query is built. Let's see how.Assume we want to find a list of
User with specific FirstName values. First, we build a regular Query with an equal() condition for FirstName. Because we have to pass an initial parameter value to equal() but plan to override it before running the Query later, we just pass an empty string:1
var caseSensitive = false
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var query = box.Query(User_.FirstName.Equals("", caseSensitive))
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Now at some later point we want to actually run the
Query. To set a value for the FirstName parameter we call setStringParams() on the Query and pass the FirstName property and the new parameter value:1
query.SetStringParams(User_.FirstName, "Joe")
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joes, _ := query.Find()
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Alias/As
So you might already be wondering, what happens if you have more than one condition using the same property? For this purpose you can assign each condition an alias by calling
Alias() right after specifying the condition:1
var query = box.Query(
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User_.Age.GreaterThan().Alias("min age"),
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User_.Age.LessThan().Alias("max age"))
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// Then use the alias when setting the parameter value
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query.SetInt64Params(objectbox.Alias("min age"), 50)
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query.SetInt64Params(objectbox.Alias("max age"), 100)
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There's also an alternative, syntax for a aliases that makes it easier to maintain the code because it avoids repeating string constants:
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var minAgeAlias = objectbox.Alias("min age")
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var maxAgeAlias = objectbox.Alias("max age")
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var query = box.Query(
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User_.Age.GreaterThan().As(minAgeAlias),
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User_.Age.LessThan().As(maxAgeAlias))
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// Then use the alias when setting the parameter value
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query.SetInt64Params(minAgeAlias, 50)
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query.SetInt64Params(maxAgeAlias, 100)
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Limit, Offset, and Pagination
Sometimes you only need a subset of a query, for example the first 10 elements. This is especially helpful (and resourceful) when you have a high number of entities and you cannot limit the result using query conditions only. The built
Query has .Offset() and .Limit() methods to help you do that1
query := box.Query(User_.FirstName.Equals("Joe", false))
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joes, err := query.Offset(10).Limit(5).Find()
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Offset(n uint64): the first n results are skipped.
Limit(n uint64): at most n results of this query are returned.Ordering results
In addition to specifying conditions you can order the returned results:
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query := box.Query(User_.FirstName.Equals("Joe", false), User_.Age.OrderDesc())
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joes, err := query.Find()
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You can combine multiple order parameters and options (some options are only available for certain data types, e.g. strings have case-sensitive ordering option), such as:
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query := box.Query(
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User_.FirstName.Equals("Joe", false),
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User_.LastName.OrderDesc(false), // caseSensitive bool argument
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User_.Age.OrderAsc()
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)
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joes, err := query.Find()
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Notable conditions/operators
In addition to expected conditions like
Equals(), NotEquals(), GreaterThan() and LessThan() there are also conditions like:Between()to filter for values that are between the given two (inclusive)In()andNotIn()to filter for values that match any in the given set,HasPrefix(),HasSuffix()andContains()for extended String filtering.
Working with query results
You have a few options how to handle the results of a query:
Find()returns a slice of the matching objects,FindIds()fetches just the IDs of the matching objects as a slice, which can be more efficient in case you don't need the whole object,Remove()deletes all the matching objects from the database (in a single transaction),Count()gives you the number of the objects that match the query,Limit()andOffset()let you select just part of the result (e. g. for paging)DescribeParams()is a utility function which returns a human-readable representation of the query.
Querying linked objects (relations)
After creating a relation between entities, you might want to add a query condition for a property that only exists in the related entity. In SQL this is solved using JOINs. ObjectBox provides query links instead.
Let's see how this works using an example.
Assume there is a
Person that can be associated with multiple Address entities:1
//go:generate go run github.com/objectbox/objectbox-go/cmd/objectbox-gogen
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​
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type Person struct {
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Id uint64
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Name string
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Address []*Address
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}
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​
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type Address struct {
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Id uint64
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Street string
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ZIP string
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}
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To get a
Person with a certain name that also lives on a specific street, we need to query the associated Address entities of a Person. To do this, use the Person_.Address.Link(cs ...Conditions) method of the generated Person_ variable to tell that the addresses relation should be queried and what conditions should be used to filter the addresses:1
// get all Person objects named "Elmo" which have an address on "Sesame Street"
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var query = BoxForPerson(ob).Query(
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Person_.name.Equals("Elmo", true),
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Person_.Address.Link(Address_.Street.Equals("Sesame Street", true)),
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)
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var elmosOnSesameStreet = query.Find()
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What if we want to get a list of
Address instead of Person? No problem, links are smart enough to know there's also an implicit relation in the opposite direction. Note the different box we're using here:1
// get all Address objects on "Sesame Street" linked from a Person named "Elmo"
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val builder = box.query().equal(Address_.Street, "Sesame Street")
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var query = BoxForAddress(ob).Query(
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Address_.Street.Equals("Sesame Street", true),
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Person_.Address.Link(Person_.Name.Equals("Elmo", true)),
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)
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var addressesSesameStreetWithElmo = query.Find()
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PropertyQuery
If you only want to return the values of a particular property and not a list of full objects you can use a PropertyQuery. After building a query, simply call
query.Property(Property) . For example, instead of getting all Users, to just get their email addresses:1
query := userBox.Query()
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emails, err := query.Property(User_.Email).FindStrings(nil)
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The returned items are not in any particular order, even if you did specify an order when building the query.
Handling null values
The argument to
FindStrings() (and similar for other types) is a value to be used if the given field is nil in the database. By default, i.e. when you pass `nil` as the argument, these values are not returned. However, you can specify a replacement value to return if a property is null:1
// includes 'unknown' instead of each null email
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emails, err := userBox.Query().Property(User_.Email).FindStrings("unknown")
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Distinct values
The property query can also only return distinct values:
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pq := userBox.Query().Property(User_.FirstName)
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​
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// returns ['joe'] because by default, the case of strings is ignored.
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err := pq.Distinct(true) // args: Distinct(value bool)
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names := pq.FindStrings(nil)
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​
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// returns ['Joe', 'joe', 'JOE']
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pq.DistinctString(true, true) // args: DistinctString(value, caseSensitive bool)
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names = pq.FindStrings(nil)
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Aggregating values
Property queries also offer aggregate functions to directly calculate the minimum, maximum, average, sum and count of all found (non-null) values:
Min()/MinDouble(): Finds the minimum value for the given property over all objects matching the query.Max()/MaxFloat64(): Finds the maximum value.Sum()/SumFloat64(): Calculates the sum of all values. Note: the integer version detects overflows and returns an error in that case.Average(): Calculates the average (always afloat64) of all values.Count(): returns the number of results. This is faster than finding and getting the length of the result array. Can be combined withDistinct()to count only the number of distinct values.
Last modified 10mo ago