30 lines
1.6 KiB
Plaintext
30 lines
1.6 KiB
Plaintext
== SQM
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The Semantic Query Model (SQM) is Hibernate's representation of an HQL or Criteria query's semantic (meaning). This
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representation is modeled as an "abstract syntax tree" (AST) - meaning it is a structured tree of nodes where each node
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represrents an atomic piece of the query. E.g. `SqmSelectClause` represents the query's select clause as you might
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imagine. That `SqmSelectClause` is ultimately a collection of one or more `SqmSelection` references representing the
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individual selections to be returned from the query (called the domain results).
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=== The Model
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This SQM model uses the Hibernate domain model, which is Hibernate's extension to the JPA model. This model contains no
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relational mapping information, it simply describes the domain model in mostly Java terms although it does incorporate
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"classifications" of the type system. E.g. it understand that `Customer` is an entity, but contains no information
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about the tables it maps to nor its columns.
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See the `design/type-system-domain.adoc` design doc. For details about this domain model
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=== Building an SQM
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=== Interpreting an SQM
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Ultimately Hibernate needs to talk with the database to fulfill these query executions. This is currently a 2-step process.
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First we convert the SQM into a new AST called the SQL AST. This is an AST that is more "SQL-y". It's nodes are defined
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in terms of Hibernate's mapping model which is the model that actually does understand the relational mapping.
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See `design/type-system-mapping.adoc` for details about this model. Part of this conversion step is to resolving
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domain model references to mapping model references...
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