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154 lines
3.9 KiB
Markdown
154 lines
3.9 KiB
Markdown
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```
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---
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authors: Evie Ciobanu <evie@hasura.io>
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state: draft
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---
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```
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# MySQL Relationships
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As the name suggests, relationships are a core feature of relational
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databases. Using relationships between entities, we can query or update
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structured data as well as ensure data consistency.
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## Description
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Since relationships are such a key feature of any relational database,
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including MySQL, the Hasura engine is expected to be able to handle
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them as well.
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Relationships are either object or array. Object relationships represent
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a single related entity (e.g., an article is only published by one
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author), whereas array relationships may have any number of related entities
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(e.g., an author may have any number of published articles).
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### Success
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This feature allows us to track, untrack, and query both object and array
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relationships for MySQL sources.
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Additionally, the `TestRelationships` test suite should pass on MySQL sources
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(https://github.com/hasura/graphql-engine-mono/blob/6905d5914c8a698445c0ef03d6a8303747701e1c/server/tests-py/test_v1_queries.py#L565)
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The following tests should also pass for MySQL sources:
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```
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# relationships
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test_select_query_multiple_columns_arr_fkey
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test_select_query_multiple_columns_obj_fkey
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test_int_as_string_offset
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test_err_neg_offset_error
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# depend on relationships: nested queries
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test_nested_select_query_article_author
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test_nested_select_query_deep
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```
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## What
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This feature translates to implementing certain instance methods for
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MySQL sources, see https://github.com/hasura/graphql-engine-mono/pull/1110
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as an example.
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It is not currently expected that any changes would be necessary in the
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common areas of the code, other than adding the appropriate API endpoints:
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- `mysql_create_object_relationship`
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- `mysql_create_array_relationship`
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- `mysql_set_relationship_comment`
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- `mysql_rename_relationship`
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- `mysql_drop_relationship`
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If a need for any non-trivial changes to the common code arises (such as,
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but not limited to adding or changing method signatures in the `Backend*`
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classes, IR code, parser code, etc.), this should be brought up with the
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data sources and the IR teams to discuss options before proceeding.
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On top of metadata-related operations, the following features are
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implied when considering relationships:
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- selecting data using relationships
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- relationships in boolean expressions
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- relationships in permission boolean expressions
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- `_aggregate` fields for array relationships
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- ordering using relationships
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The above features are listed from most to least important.
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### Selecting data using relationships
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It is expected that creating a relationship creates a new field that
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is selectable. For example, getting the author of a published article
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by using`articles.author`.
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```
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query {
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articles {
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id
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title
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author {
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id
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name
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}
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}
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}
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```
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Relevant test: `test_nested_select_query_article_author`
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### Relationships in boolean expressions
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Filtering should work on relationships, for example, we should be able
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to filter articles by their author's name:
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```
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query {
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articles (where: {author: {name: {_eq: "Sidney"}}}) {
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id
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title
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}
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}
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```
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Relevant test: `test_nested_select_query_where_on_relationship`
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### Relationships in permission boolean expressions
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Permissions should also be usable in boolean expressions. For more
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details, please see the MySQL Permissions RFC.
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### Aggregate fields for array relationshipsd
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It is expected that each array relationship creates a `name_aggregate`
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field:
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```
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query {
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autors {
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articles_aggregate {
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aggregate {
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count
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}
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}
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}
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}
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```
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Relevant test: `test_author_agg_with_articles`
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### Ordering using relationships
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Nested objects should be usable in ordering clauses.
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```
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query {
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articles( order_by: { author: { id: desc } } ) {
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id
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name
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}
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}
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```
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Relevant test: `test_nested_select_query_where_on_relationship`
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## Effects and Interactions
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This feature will likely require support from the Console and CLI teams.
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