---
title: "Update to `filter()` behaviour in bcdata v0.4.0"
date: "2024-12-11"
output: rmarkdown::html_vignette
vignette: >
  %\VignetteIndexEntry{Update to `filter()` behaviour in bcdata v0.4.0}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---

This vignette describes a change in `{bcdata}` v0.4.0 related to 
using locally-executed functions in a `filter()` query with `bcdc_query_geodata()`:

When using `bcdc_query_geodata()` with `filter()`, many functions are
translated to a query plan that is passed to and executed on the server - this includes the
CQL Geometry predicates such as `INTERESECTS()`, `CROSSES()`, `BBOX()`
etc, as well as many base R functions. However you sometimes want to
include a function call in your `filter()` statement which should be
evaluated locally - i.e., it's an R function (often an `{sf}` function)
with no equivalent function on the server. Prior to version 0.4.0,
`{bcdata}` did a reasonable (though not perfect) job of detecting R
functions inside a `filter()` statement that needed to be evaluated
locally. In order to align with recommended best practices for
`{dbplyr}` backends, as of v0.4.0, function calls that are to be
evaluated locally now need to be wrapped in `local()`.

For example, say we want to create a bounding box around two points and
use that box to perform a spatial filter on the remote dataset, to
give us just the set of local greenspaces that exist within that
bounding box.





``` r
library(sf)
library(bcdata)

two_points <- st_sfc(st_point(c(1164434, 368738)),
                     st_point(c(1203023, 412959)),
                     crs = 3005)
```

Previously, we could just do this, with `sf::st_bbox()`
embedded in the call:


``` r
bcdc_query_geodata("local-and-regional-greenspaces") %>%
  filter(BBOX(st_bbox(two_points, crs = st_crs(two_points))))
```

```
## Error: Error : Cannot translate a <sfc_POINT> object to SQL.
## ℹ Do you want to force evaluation in R with (e.g.) `!!x` or `local(x)`?
```

However you must now use `local()` to force local evaluation of
`st_bbox()` on your machine in R, before it is translated
into a query plan to be executed on the server:


``` r
bcdc_query_geodata("local-and-regional-greenspaces") %>%
  filter(BBOX(local(st_bbox(two_points, crs = st_crs(two_points)))))
```

```
## Querying 'local-and-regional-greenspaces' record
## • Using collect() on this object will return 1158 features and 19 fields
## • At most six rows of the record are printed here
## ────────────────────────────────────────────────────────────────────────────────────────────────────
## Simple feature collection with 6 features and 19 fields
## Geometry type: POLYGON
## Dimension:     XY
## Bounding box:  xmin: 1200113 ymin: 385903.5 xmax: 1202130 ymax: 388026
## Projected CRS: NAD83 / BC Albers
## # A tibble: 6 × 20
##   id      LOCAL_REG_GREENSPACE…¹ PARK_NAME PARK_TYPE PARK_PRIMARY_USE REGIONAL_DISTRICT MUNICIPALITY
##   <chr>                    <int> <chr>     <chr>     <chr>            <chr>             <chr>       
## 1 WHSE_B…                    689 Cranford… Local     Water Access     Capital           District of…
## 2 WHSE_B…                    634 <NA>      Local     Water Access     Capital           District of…
## 3 WHSE_B…                    725 <NA>      Local     Water Access     Capital           District of…
## 4 WHSE_B…                    665 Konukson… Local     Green Space      Capital           District of…
## 5 WHSE_B…                    622 <NA>      Local     Trail            Capital           District of…
## 6 WHSE_B…                    698 <NA>      Local     Water Access     Capital           District of…
## # ℹ abbreviated name: ¹​LOCAL_REG_GREENSPACE_ID
## # ℹ 13 more variables: CIVIC_NUMBER <chr>, CIVIC_NUMBER_SUFFIX <chr>, STREET_NAME <chr>,
## #   LATITUDE <dbl>, LONGITUDE <dbl>, WHEN_UPDATED <date>, WEBSITE_URL <chr>,
## #   LICENCE_COMMENTS <chr>, FEATURE_AREA_SQM <dbl>, FEATURE_LENGTH_M <dbl>, OBJECTID <int>,
## #   SE_ANNO_CAD_DATA <chr>, geometry <POLYGON [m]>
```

There is another illustration in the ["querying spatial data vignette"](https://bcgov.github.io/bcdata/articles/efficiently-query-spatial-data-in-the-bc-data-catalogue.html#a-note-about-using-local-r-functions-in-constructing-filter-queries).