Michael Bayne
67 lines
1.9 KiB
Markdown
67 lines
1.9 KiB
Markdown
_Overview of Computed Records._
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Note: computed records are largely deprecated in favor of ad-hoc queries or the use of
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`selectInto`. See [the example queries page](ExampleQueries) for examples of such use.
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## Computed Records
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You can easily define record with computed fields or records that represent a join across multiple
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tables (see below). It is very easy to select a subset of a record's fields:
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```java
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@Entity @Computed(shadowOf=PersonRecord.class)
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public PersonNameRecord extends PersistentRecord
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{
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public int personId;
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public String name;
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}
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List<PersonNameRecord> allNames = findAll(PersonNameRecord.class);
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List<PersonNameRecord> youngNames = findAll(
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PersonNameRecord.class, new Where(PersistentRecord.AGE.lessEq(18)));
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```
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You can also define computed records that calculate information:
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```java
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@Entity @Computed
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public CountRecord extends PersistentRecord
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{
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@Computed(fieldDefinition="count(*)")
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public int count;
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}
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int personCount = load(CountRecord.class, new FromOverride(PersonRecord.class)).count;
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// or if you want to be less general purpose
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@Entity @Computed(shadowOf=PersonRecord.class)
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public PersonCountRecord extends PersistentRecord
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{
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@Computed(fieldDefinition="count(*)")
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public int count;
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}
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int personCount = load(PersonCountRecord.class).count;
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// or something more specific
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@Entity @Computed(shadowOf=PersonRecord.class)
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public PersonAvgAgeRecord extends PersistentRecord
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{
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@Computed(fieldDefinition="avg(*)")
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public float averageAge;
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}
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float averageAge = load(PersonAvgAgeRecord.class).averageAge;
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```
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The reader may be alarmed to notice some hard-coded SQL in those classes. One is advised to stick
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to very standard SQL in cases like this to avoid introducing portability problems, but we didn't
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think it was worth the trouble to try to model these simple and mostly standard operations in a
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more complex way. YMMV.
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## Joins with Computed Records
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TBD
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