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36 lines
1.2 KiB
Markdown
36 lines
1.2 KiB
Markdown
# Add A Check Constraint To A Table
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PostgreSQL allows you to enforce all kinds of rules about the value of a column
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or the relationship between two columns. These rules are defined with [_check
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constraints_](https://www.postgresql.org/docs/current/ddl-constraints.html#DDL-CONSTRAINTS-CHECK-CONSTRAINTS).
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ActiveRecord's migration DSL does not provide a way for adding check
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constraints directly. They can be added by executing a SQL statement in a
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migration.
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```ruby
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class EnsurePageCountIsPositive < ActiveRecord::Migration[5.2]
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def up
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execute <<-SQL
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alter table books
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add constraint ensure_page_count_is_positive
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check (page_count > 0);
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SQL
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end
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def down
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execute <<-SQL
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alter table books
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drop constraint ensure_page_count_is_positive;
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SQL
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end
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```
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This check constraint ensures that, anytime you add or update a row in the book
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column, the value of `page_count` column is always greater than `0`. This is a
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nice thing to enforce because it wouldn't make much sense for a book to have,
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say, `-10` pages.
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Note: these constraints will not appear in your `db/schema.rb` file. If you
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want to see what check constraints have been defined across your tables, you
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can crack open `psql` to investigate.
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