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Avoid coercing all SELECTs with the same column name to the same value #36186

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Avoid coercing all SELECTs with the same column name to the same value #36186

wants to merge 4 commits into from

Commits on Apr 22, 2020

  1. Avoid coercing all SELECTs with the same column name to the same value 

    Fixes rails#36042

Rails had been mapping types to columns in a Result Set by indexing types on columns' names. When two values in a Result Set have the same column name, they are forced to share a type.

There are several scenarios where multiple values may share the same column name.

1. One (where two tables happen to have columns with the same name but the column's values differ) is outlined in rails#36042.

2. Another is when using functions in a `.select` or `.pluck`. In the absence of an Alias clause, Postgres names values with the last function called, so if two values in the same query happen to use `COALESCE`, they'll both be named `"coalesce"`.

   ```ruby
   ActiveRecord::Base.pluck(
     Arel.sql("COALESCE(NULL, 'four')"),
     Arel.sql("COALESCE(NULL, 4)")
   )
   ```

This commit matches `types` with `columns` and `values` (in a row) by array position (which is an assumption already made in `PostgreSQL::DatabaseStatements#exec_query`). It adds `column_types` as a public method on `ActiveRecord::Result` to preserve the class's public API.

Does Rails expect client code to be constructing `ActiveRecord::Result` objects? Do we need to check whether its third argument is a `Hash`, give — maybe — a deprecation warning and convert it to an `Array`?
    @boblail
    boblail committed Apr 22, 2020
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  2. fixup! renamed :column_types to :types in build_result

    @boblail
    boblail committed Apr 22, 2020
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  3. fixup! add test for pluck that shows what public behavior this change… 

    … fixes
    @boblail
    boblail committed Apr 22, 2020
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  4. fixup! precalculated @column_types

    @boblail
    boblail committed Apr 22, 2020
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