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<img src="" class="logo" alt=""><h1>Metadata</h1>
<small class="dont-index">Source: <a href="https://github.com/apache/arrow/blob/main/r/vignettes/metadata.Rmd" class="external-link"><code>vignettes/metadata.Rmd</code></a></small>
<div class="d-none name"><code>metadata.Rmd</code></div>
</div>
<p>This article describes the various data and metadata object types supplied by arrow, and documents how these objects are structured.</p>
<div class="section level2">
<h2 id="arrow-metadata-classes">Arrow metadata classes<a class="anchor" aria-label="anchor" href="#arrow-metadata-classes"></a>
</h2>
<p>The arrow package defines the following classes for representing metadata:</p>
<ul>
<li>A <code>Schema</code> is a list of <code>Field</code> objects used to describe the structure of a tabular data object; where</li>
<li>A <code>Field</code> specifies a character string name and a <code>DataType</code>; and</li>
<li>A <code>DataType</code> is an attribute controlling how values are represented</li>
</ul>
<p>Consider this:</p>
<div class="sourceCode" id="cb1"><pre class="downlit sourceCode r">
<code class="sourceCode R"><span><span class="va">df</span> <span class="op">&lt;-</span> <span class="fu"><a href="https://rdrr.io/r/base/data.frame.html" class="external-link">data.frame</a></span><span class="op">(</span>x <span class="op">=</span> <span class="fl">1</span><span class="op">:</span><span class="fl">3</span>, y <span class="op">=</span> <span class="fu"><a href="https://rdrr.io/r/base/c.html" class="external-link">c</a></span><span class="op">(</span><span class="st">"a"</span>, <span class="st">"b"</span>, <span class="st">"c"</span><span class="op">)</span><span class="op">)</span></span>
<span><span class="va">tb</span> <span class="op">&lt;-</span> <span class="fu"><a href="../reference/table.html">arrow_table</a></span><span class="op">(</span><span class="va">df</span><span class="op">)</span></span>
<span><span class="va">tb</span><span class="op">$</span><span class="va">schema</span></span></code></pre></div>
<pre><code><span><span class="co">## Schema</span></span>
<span><span class="co">## x: int32</span></span>
<span><span class="co">## y: string</span></span>
<span><span class="co">## </span></span>
<span><span class="co">## See $metadata for additional Schema metadata</span></span></code></pre>
<p>The schema that has been automatically inferred could also be manually created:</p>
<div class="sourceCode" id="cb3"><pre class="downlit sourceCode r">
<code class="sourceCode R"><span><span class="fu"><a href="../reference/schema.html">schema</a></span><span class="op">(</span></span>
<span> <span class="fu"><a href="../reference/Field.html">field</a></span><span class="op">(</span>name <span class="op">=</span> <span class="st">"x"</span>, type <span class="op">=</span> <span class="fu"><a href="../reference/data-type.html">int32</a></span><span class="op">(</span><span class="op">)</span><span class="op">)</span>,</span>
<span> <span class="fu"><a href="../reference/Field.html">field</a></span><span class="op">(</span>name <span class="op">=</span> <span class="st">"y"</span>, type <span class="op">=</span> <span class="fu"><a href="../reference/data-type.html">utf8</a></span><span class="op">(</span><span class="op">)</span><span class="op">)</span></span>
<span><span class="op">)</span></span></code></pre></div>
<pre><code><span><span class="co">## Schema</span></span>
<span><span class="co">## x: int32</span></span>
<span><span class="co">## y: string</span></span></code></pre>
<p>The <code><a href="../reference/schema.html">schema()</a></code> function allows the following shorthand to define fields:</p>
<div class="sourceCode" id="cb5"><pre class="downlit sourceCode r">
<code class="sourceCode R"><span><span class="fu"><a href="../reference/schema.html">schema</a></span><span class="op">(</span>x <span class="op">=</span> <span class="fu"><a href="../reference/data-type.html">int32</a></span><span class="op">(</span><span class="op">)</span>, y <span class="op">=</span> <span class="fu"><a href="../reference/data-type.html">utf8</a></span><span class="op">(</span><span class="op">)</span><span class="op">)</span></span></code></pre></div>
<pre><code><span><span class="co">## Schema</span></span>
<span><span class="co">## x: int32</span></span>
<span><span class="co">## y: string</span></span></code></pre>
<p>Sometimes it is important to specify the schema manually, particularly if you want fine-grained control over the Arrow data types:</p>
<div class="sourceCode" id="cb7"><pre class="downlit sourceCode r">
<code class="sourceCode R"><span><span class="fu"><a href="../reference/table.html">arrow_table</a></span><span class="op">(</span><span class="va">df</span>, schema <span class="op">=</span> <span class="fu"><a href="../reference/schema.html">schema</a></span><span class="op">(</span>x <span class="op">=</span> <span class="fu"><a href="../reference/data-type.html">int64</a></span><span class="op">(</span><span class="op">)</span>, y <span class="op">=</span> <span class="fu"><a href="../reference/data-type.html">utf8</a></span><span class="op">(</span><span class="op">)</span><span class="op">)</span><span class="op">)</span></span></code></pre></div>
<pre><code><span><span class="co">## Table</span></span>
<span><span class="co">## 3 rows x 2 columns</span></span>
<span><span class="co">## $x &lt;int64&gt;</span></span>
<span><span class="co">## $y &lt;string&gt;</span></span>
<span><span class="co">## </span></span>
<span><span class="co">## See $metadata for additional Schema metadata</span></span></code></pre>
<div class="sourceCode" id="cb9"><pre class="downlit sourceCode r">
<code class="sourceCode R"><span><span class="fu"><a href="../reference/table.html">arrow_table</a></span><span class="op">(</span><span class="va">df</span>, schema <span class="op">=</span> <span class="fu"><a href="../reference/schema.html">schema</a></span><span class="op">(</span>x <span class="op">=</span> <span class="fu"><a href="../reference/data-type.html">float64</a></span><span class="op">(</span><span class="op">)</span>, y <span class="op">=</span> <span class="fu"><a href="../reference/data-type.html">utf8</a></span><span class="op">(</span><span class="op">)</span><span class="op">)</span><span class="op">)</span></span></code></pre></div>
<pre><code><span><span class="co">## Table</span></span>
<span><span class="co">## 3 rows x 2 columns</span></span>
<span><span class="co">## $x &lt;double&gt;</span></span>
<span><span class="co">## $y &lt;string&gt;</span></span>
<span><span class="co">## </span></span>
<span><span class="co">## See $metadata for additional Schema metadata</span></span></code></pre>
</div>
<div class="section level2">
<h2 id="r-object-attributes">R object attributes<a class="anchor" aria-label="anchor" href="#r-object-attributes"></a>
</h2>
<p>Arrow supports custom key-value metadata attached to Schemas. When we convert a <code>data.frame</code> to an Arrow Table or RecordBatch, the package stores any <code><a href="https://rdrr.io/r/base/attributes.html" class="external-link">attributes()</a></code> attached to the columns of the <code>data.frame</code> in the Arrow object Schema. Attributes added to objects in this fashion are stored under the <code>r</code> key, as shown below:</p>
<div class="sourceCode" id="cb11"><pre class="downlit sourceCode r">
<code class="sourceCode R"><span><span class="co"># data frame with custom metadata</span></span>
<span><span class="va">df</span> <span class="op">&lt;-</span> <span class="fu"><a href="https://rdrr.io/r/base/data.frame.html" class="external-link">data.frame</a></span><span class="op">(</span>x <span class="op">=</span> <span class="fl">1</span><span class="op">:</span><span class="fl">3</span>, y <span class="op">=</span> <span class="fu"><a href="https://rdrr.io/r/base/c.html" class="external-link">c</a></span><span class="op">(</span><span class="st">"a"</span>, <span class="st">"b"</span>, <span class="st">"c"</span><span class="op">)</span><span class="op">)</span></span>
<span><span class="fu"><a href="https://rdrr.io/r/base/attr.html" class="external-link">attr</a></span><span class="op">(</span><span class="va">df</span>, <span class="st">"df_meta"</span><span class="op">)</span> <span class="op">&lt;-</span> <span class="st">"custom data frame metadata"</span></span>
<span><span class="fu"><a href="https://rdrr.io/r/base/attr.html" class="external-link">attr</a></span><span class="op">(</span><span class="va">df</span><span class="op">$</span><span class="va">y</span>, <span class="st">"col_meta"</span><span class="op">)</span> <span class="op">&lt;-</span> <span class="st">"custom column metadata"</span></span>
<span></span>
<span><span class="co"># when converted to a Table, the metadata is preserved</span></span>
<span><span class="va">tb</span> <span class="op">&lt;-</span> <span class="fu"><a href="../reference/table.html">arrow_table</a></span><span class="op">(</span><span class="va">df</span><span class="op">)</span></span>
<span><span class="va">tb</span><span class="op">$</span><span class="va">metadata</span></span></code></pre></div>
<pre><code><span><span class="co">## $r</span></span>
<span><span class="co">## $r$attributes</span></span>
<span><span class="co">## $r$attributes$df_meta</span></span>
<span><span class="co">## [1] "custom data frame metadata"</span></span>
<span><span class="co">## </span></span>
<span><span class="co">## </span></span>
<span><span class="co">## $r$columns</span></span>
<span><span class="co">## $r$columns$x</span></span>
<span><span class="co">## NULL</span></span>
<span><span class="co">## </span></span>
<span><span class="co">## $r$columns$y</span></span>
<span><span class="co">## $r$columns$y$attributes</span></span>
<span><span class="co">## $r$columns$y$attributes$col_meta</span></span>
<span><span class="co">## [1] "custom column metadata"</span></span>
<span><span class="co">## </span></span>
<span><span class="co">## </span></span>
<span><span class="co">## $r$columns$y$columns</span></span>
<span><span class="co">## NULL</span></span></code></pre>
<p>It is also possible to assign additional string metadata under any other key you wish, using a command like this:</p>
<div class="sourceCode" id="cb13"><pre class="downlit sourceCode r">
<code class="sourceCode R"><span><span class="va">tb</span><span class="op">$</span><span class="va">metadata</span><span class="op">$</span><span class="va">new_key</span> <span class="op">&lt;-</span> <span class="st">"new value"</span></span></code></pre></div>
<p>Metadata attached to a Schema is preserved when writing the Table to Arrow/Feather or Parquet formats. When reading those files into R, or when calling <code><a href="https://rdrr.io/r/base/as.data.frame.html" class="external-link">as.data.frame()</a></code> on a Table or RecordBatch, the column attributes are restored to the columns of the resulting <code>data.frame</code>. This means that custom data types, including <code>haven::labelled</code>, <code>vctrs</code> annotations, and others, are preserved when doing a round-trip through Arrow.</p>
<p>Note that the attributes stored in <code>$metadata$r</code> are only understood by R. If you write a <code>data.frame</code> with <code>haven</code> columns to a Feather file and read that in Pandas, the <code>haven</code> metadata won’t be recognized there. Similarly, Pandas writes its own custom metadata, which the R package does not consume. You are free, however, to define custom metadata conventions for your application and assign any (string) values you want to other metadata keys.</p>
</div>
<div class="section level2">
<h2 id="further-reading">Further reading<a class="anchor" aria-label="anchor" href="#further-reading"></a>
</h2>
<ul>
<li>To learn more about arrow metadata, see the documentation for <code><a href="../reference/schema.html">schema()</a></code>.</li>
<li>To learn more about data types, see the <a href="./data_types.html">data types article</a>.</li>
</ul>
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