@@ -127,30 +127,64 @@ class CategoriesModel
 /**
 * Fetches details of every category associated with selected depictions, converts them into
 * CategoryItem and returns them in a list.
+* If a selected depiction has no categories, the categories in which its P18 belongs are
+* returned in the list.
 *
 * @param selectedDepictions selected DepictItems
 * @return List of CategoryItem associated with selected depictions
 */
-private fun categoriesFromDepiction(selectedDepictions: List<DepictedItem>): Observable<MutableList<CategoryItem>>? =
-Observable
-.fromIterable(
-selectedDepictions.map { it.commonsCategories }.flatten(),
-).map { categoryItem ->
-categoryClient
-.getCategoriesByName(
-categoryItem.name,
-categoryItem.name,
-SEARCH_CATS_LIMIT,
-).map {
-CategoryItem(
-it[0].name,
-it[0].description,
-it[0].thumbnail,
-it[0].isSelected,
-)
-}.blockingGet()
-}.toList()
-.toObservable()
+private fun categoriesFromDepiction(selectedDepictions: List<DepictedItem>): Observable<MutableList<CategoryItem>>? {
+val observables = selectedDepictions.map { depictedItem ->
+if (depictedItem.commonsCategories.isEmpty()) {
+if (depictedItem.primaryImage == null) {
+return@map Observable.just(emptyList<CategoryItem>())
+}
+Observable.just(
+depictedItem.primaryImage
+).map { image ->
+categoryClient
+.getCategoriesOfImage(
+image,
+SEARCH_CATS_LIMIT,
+).map {
+it.map { category ->
+CategoryItem(
+category.name,
+category.description,
+category.thumbnail,
+category.isSelected,
+)
+}
+}.blockingGet()
+}.flatMapIterable { it }.toList()
+.toObservable()
+} else {
+Observable
+.fromIterable(
+depictedItem.commonsCategories,
+).map { categoryItem ->
+categoryClient
+.getCategoriesByName(
+categoryItem.name,
+categoryItem.name,
+SEARCH_CATS_LIMIT,
+).map {
+CategoryItem(
+it[0].name,
+it[0].description,
+it[0].thumbnail,
+it[0].isSelected,
+)
+}.blockingGet()
+}.toList()
+.toObservable()
+}
+}
+return Observable.concat(observables)
+.scan(mutableListOf<CategoryItem>()) { accumulator, currentList ->
+accumulator.apply { addAll(currentList) }
+}
+}
 
 /**
 * Fetches details of every category by their name, converts them into

@@ -78,6 +78,24 @@ class CategoryClient
 ),
 )
 
+/**
+* Fetches categories belonging to an image (P18 of some wikidata entity).
+*
+* @param image P18 of some wikidata entity
+* @param itemLimit How many categories to return
+* @return Single Observable emitting the list of categories
+*/
+fun getCategoriesOfImage(
+image: String,
+itemLimit: Int,
+): Single<List<CategoryItem>> =
+responseMapper(
+categoryInterface.getCategoriesByTitles(
+"File:${image}",
+itemLimit,
+),
+)
+
 /**
 * The method takes categoryName as input and returns a List of Subcategories
 * It uses the generator query API to get the subcategories in a category, 500 at a time.

@@ -61,6 +61,21 @@ interface CategoryInterface {
 @Query("gacoffset") offset: Int,
 ): Single<MwQueryResponse>
 
+/**
+* Fetches non-hidden categories by titles.
+*
+* @param titles titles to fetch categories for (e.g. File:<P18 of a wikidata entity>)
+* @param itemLimit How many categories to return
+* @return MwQueryResponse
+*/
+@GET(
+"w/api.php?action=query&format=json&formatversion=2&generator=categories&prop=categoryinfo|description|pageimages&piprop=thumbnail&pithumbsize=70&gclshow=!hidden",
+)
+fun getCategoriesByTitles(
+@Query("titles") titles: String?,
+@Query("gcllimit") itemLimit: Int,
+): Single<MwQueryResponse>
+
 @GET("w/api.php?action=query&format=json&formatversion=2&generator=categorymembers&gcmtype=subcat&prop=info&gcmlimit=50")
 fun getSubCategoryList(
 @Query("gcmtitle") categoryName: String,

@@ -68,6 +68,9 @@ data class DepictedItem constructor(
 entity.id(),
 )
 
+val primaryImage: String?
+get() = imageUrl?.split('-')?.lastOrNull()
+
 override fun equals(other: Any?) =
 when {
 this === other -> true

@@ -1,7 +1,9 @@
 package fr.free.nrw.commons.category
 
 import categoryItem
+import com.nhaarman.mockitokotlin2.any
 import com.nhaarman.mockitokotlin2.mock
+import com.nhaarman.mockitokotlin2.times
 import com.nhaarman.mockitokotlin2.verify
 import com.nhaarman.mockitokotlin2.whenever
 import depictedItem
@@ -90,14 +92,18 @@ class CategoriesModelTest {
 val depictedItem =
 depictedItem(
 commonsCategories =
-listOf(
-CategoryItem(
-"depictionCategory",
-"",
-"",
-false,
-),
+listOf(
+CategoryItem(
+"depictionCategory",
+"",
+"",
+false,
 ),
+),
+)
+val depictedItemWithoutCategories =
+depictedItem(
+imageUrl = "testUrl"
 )
 
 whenever(gpsCategoryModel.categoriesFromLocation)
@@ -159,6 +165,23 @@ class CategoriesModelTest {
 ),
 ),
 )
+whenever(
+categoryClient.getCategoriesOfImage(
+"testUrl",
+25,
+),
+).thenReturn(
+Single.just(
+listOf(
+CategoryItem(
+"categoriesOfP18",
+"",
+"",
+false,
+),
+),
+),
+)
 val imageTitleList = listOf("Test")
 CategoriesModel(categoryClient, categoryDao, gpsCategoryModel)
 .searchAll("", imageTitleList, listOf(depictedItem))
@@ -171,8 +194,21 @@ class CategoriesModelTest {
 categoryItem("recentCategories"),
 ),
 )
+CategoriesModel(categoryClient, categoryDao, gpsCategoryModel)
+.searchAll("", imageTitleList, listOf(depictedItemWithoutCategories))
+.test()
+.assertValue(
+listOf(
+categoryItem("categoriesOfP18"),
+categoryItem("gpsCategory"),
+categoryItem("titleSearch"),
+categoryItem("recentCategories"),
+),
+)
 imageTitleList.forEach {
-verify(categoryClient).searchCategories(it, CategoriesModel.SEARCH_CATS_LIMIT)
+verify(categoryClient, times(2)).searchCategories(it, CategoriesModel.SEARCH_CATS_LIMIT)
+verify(categoryClient).getCategoriesByName(any(), any(), any(), any())
+verify(categoryClient).getCategoriesOfImage(any(), any())
 }
 }
 

@@ -132,6 +132,45 @@ class CategoryClientTest {
 )
 }
 
+@Test
+fun getCategoriesByTitlesFound() {
+val mockResponse = withMockResponse("Category:Test")
+whenever(
+categoryInterface.getCategoriesByTitles(
+anyString(),
+anyInt(),
+),
+).thenReturn(Single.just(mockResponse))
+categoryClient
+.getCategoriesOfImage("tes", 10)
+.test()
+.assertValues(
+listOf(
+CategoryItem(
+"Test",
+"",
+"",
+false,
+),
+),
+)
+categoryClient
+.getCategoriesOfImage(
+"tes",
+10,
+).test()
+.assertValues(
+listOf(
+CategoryItem(
+"Test",
+"",
+"",
+false,
+),
+),
+)
+}
+
 @Test
 fun getCategoriesByNameNull() {
 val mockResponse = withNullPages()
@@ -160,6 +199,29 @@ class CategoryClientTest {
 .assertValues(emptyList())
 }
 
+@Test
+fun getCategoriesByTitlesNull() {
+val mockResponse = withNullPages()
+whenever(
+categoryInterface.getCategoriesByTitles(
+anyString(),
+anyInt(),
+),
+).thenReturn(Single.just(mockResponse))
+categoryClient
+.getCategoriesOfImage(
+"tes",
+10,
+).test()
+.assertValues(emptyList())
+categoryClient
+.getCategoriesOfImage(
+"tes",
+10,
+).test()
+.assertValues(emptyList())
+}
+
 @Test
 fun getParentCategoryListFound() {
 val mockResponse = withMockResponse("Category:Test")

@@ -181,4 +181,20 @@ class DepictedItemTest {
 fun `hashCode returns different values for objects with different name`() {
 Assert.assertNotEquals(depictedItem(name = "a").hashCode(), depictedItem(name = "b").hashCode())
 }
+
+@Test
+fun `primaryImage is derived correctly from imageUrl`() {
+Assert.assertEquals(
+DepictedItem(
+entity(
+statements = mapOf(
+WikidataProperties.IMAGE.propertyName to listOf(
+statement(snak(dataValue = valueString("prefix: example_image name"))),
+),
+),
+),
+).primaryImage,
+"_example_image_name",
+)
+}
 }